diff --git a/backend/prisma/migrations/20260927150000_add_invitations/migration.sql b/backend/prisma/migrations/20260927150000_add_invitations/migration.sql
new file mode 100644
index 0000000..47578b8
--- /dev/null
+++ b/backend/prisma/migrations/20260927150000_add_invitations/migration.sql
@@ -0,0 +1,21 @@
+-- CreateTable
+CREATE TABLE "Invitation" (
+    "id" TEXT NOT NULL,
+    "email" TEXT NOT NULL,
+    "token" TEXT NOT NULL,
+    "invitedById" TEXT,
+    "expiresAt" TIMESTAMP(3) NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "Invitation_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Invitation_email_key" ON "Invitation"("email");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Invitation_token_key" ON "Invitation"("token");
+
+-- AddForeignKey
+ALTER TABLE "Invitation" ADD CONSTRAINT "Invitation_invitedById_fkey" FOREIGN KEY ("invitedById") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;
diff --git a/backend/prisma/schema.prisma b/backend/prisma/schema.prisma
index eac0245..b64bcb9 100644
--- a/backend/prisma/schema.prisma
+++ b/backend/prisma/schema.prisma
@@ -58,10 +58,27 @@ model User {
   logs                     Log[]
   authorLinks              AuthorLink[]
   bookmarks                Bookmark[]
+  invitationsSent          Invitation[]
   createdAt                DateTime       @default(now())
   updatedAt                DateTime       @updatedAt
 }
 
+// A pending invitation to register while new registrations are turned off (Administration >
+// Users). The emailed link carries `token`; registering with it is the only way in for
+// that email, and it's deleted the moment the account is created. One per email -- inviting the
+// same address again replaces the token, so only the newest link works. See
+// services/invitationService.ts.
+model Invitation {
+  id          String   @id @default(cuid())
+  email       String   @unique
+  token       String   @unique
+  invitedById String?
+  invitedBy   User?    @relation(fields: [invitedById], references: [id], onDelete: SetNull)
+  expiresAt   DateTime
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+}
+
 model Category {
   id                String     @id @default(cuid())
   userId            String
@@ -101,18 +118,18 @@ model Category {
 }
 
 model Print {
-  id               String           @id @default(cuid())
+  id               String                   @id @default(cuid())
   userId           String
-  user             User             @relation(fields: [userId], references: [id], onDelete: Cascade)
+  user             User                     @relation(fields: [userId], references: [id], onDelete: Cascade)
   name             String
   nameNormalized   String
   title            String?
   notes            String?
   creator          String?
   collection       String?
-  tags             String[]         @default([])
-  viewCount        Int              @default(0)
-  printCount       Int              @default(0)
+  tags             String[]                 @default([])
+  viewCount        Int                      @default(0)
+  printCount       Int                      @default(0)
   // Backs the two built-in "Favourites"/"Browsing History" pseudo-collections (see
   // routes/collections.ts's SYSTEM_COLLECTIONS) -- not a real Collection/CollectionItem row.
   // lastViewedAt is stamped on every GET /print/:id alongside viewCount, so "Browsing History"
@@ -126,19 +143,19 @@ model Print {
   sourceProvider   String?
   sourceExternalId String?
   categoryId       String?
-  category         Category?        @relation(fields: [categoryId], references: [id], onDelete: SetNull)
+  category         Category?                @relation(fields: [categoryId], references: [id], onDelete: SetNull)
   authorId         String?
-  author           Author?          @relation(fields: [authorId], references: [id], onDelete: SetNull)
+  author           Author?                  @relation(fields: [authorId], references: [id], onDelete: SetNull)
   sourcePath       String?
   plates           Plate[]
   previewImages    PreviewImage[]
-  files            PrintFile[]      @relation("PrintFiles")
-  preparedFileId   String?          @unique
-  preparedFile     PrintFile?       @relation("PreparedFile", fields: [preparedFileId], references: [id], onDelete: SetNull)
+  files            PrintFile[]              @relation("PrintFiles")
+  preparedFileId   String?                  @unique
+  preparedFile     PrintFile?               @relation("PreparedFile", fields: [preparedFileId], references: [id], onDelete: SetNull)
   preparedMetadata Json?
   collectionItems  CollectionItem[]
-  createdAt        DateTime         @default(now())
-  updatedAt        DateTime         @updatedAt
+  createdAt        DateTime                 @default(now())
+  updatedAt        DateTime                 @updatedAt
   // Generated column (name/title weight A, tags weight B, notes weight C, creator weight D) that
   // backs the global search box -- see services/searchService.ts and the hand-written migration
   // that actually creates it (`GENERATED ALWAYS AS (...) STORED`, plus its own GIN index).
@@ -162,17 +179,17 @@ model Print {
 /// on nameNormalized so re-importing the same MakerWorld collection reuses the same row
 /// instead of creating a duplicate.
 model Collection {
-  id             String           @id @default(cuid())
+  id             String                   @id @default(cuid())
   userId         String
-  user           User             @relation(fields: [userId], references: [id], onDelete: Cascade)
+  user           User                     @relation(fields: [userId], references: [id], onDelete: Cascade)
   name           String
   nameNormalized String
   description    String?
-  tags           String[]         @default([])
+  tags           String[]                 @default([])
   items          CollectionItem[]
   bookmarks      Bookmark[]
-  createdAt      DateTime         @default(now())
-  updatedAt      DateTime         @updatedAt
+  createdAt      DateTime                 @default(now())
+  updatedAt      DateTime                 @updatedAt
   // Same generated-tsvector search column as Print.searchVector above (name weight A, tags
   // weight B, description weight C) -- see that field's comment and services/searchService.ts.
   searchVector   Unsupported("tsvector")?
@@ -302,21 +319,21 @@ model Notification {
 /// duplicate. Print.creator stays as the plain display string for backward compatibility and
 /// non-provider sources; this table is the richer record clickable author pages will use.
 model Author {
-  id            String   @id
+  id            String      @id
   provider      String
   externalId    String
   name          String?
   handle        String?
   bio           String?
   bioTranslated String?
-  links         String[] @default([])
+  links         String[]    @default([])
   avatarUrl     String?
   backgroundUrl String?
   prints        Print[]
   // At most one Thingport account can claim to be this real-world creator -- see AuthorLink.
   link          AuthorLink?
-  createdAt     DateTime @default(now())
-  updatedAt     DateTime @updatedAt
+  createdAt     DateTime    @default(now())
+  updatedAt     DateTime    @updatedAt
 
   @@unique([provider, externalId])
 }
diff --git a/backend/src/routes/admin.ts b/backend/src/routes/admin.ts
index 027d1d6..acc8bbb 100644
--- a/backend/src/routes/admin.ts
+++ b/backend/src/routes/admin.ts
@@ -3,7 +3,11 @@ import { prisma } from "../db";
 import { requireAdmin, requireAuth } from "../auth";
 import { HttpError } from "../utils/fileUtils";
 import { asyncHandler } from "../utils/asyncHandler";
+import { z } from "zod";
+import { parseBody } from "../utils/validate";
 import { deleteAllPrintsForUser, getStorageUsage, listLogs, listUsersWithPrintCounts } from "../services/adminService";
+import { createLog } from "../services/auditLog";
+import { INVITATION_TTL_DAYS, inviteUser } from "../services/invitationService";
 
 const router = Router();
 router.use(requireAuth);
@@ -67,6 +71,27 @@ router.get(
   }),
 );
 
+// Administration > Users > "Invite users" -- only while registrations are closed and SMTP
+// is set up (inviteUser enforces both). Inviting an already-invited address sends a fresh link
+// and retires the old one.
+const inviteSchema = z.object({ email: z.string().trim().email("Enter a valid email address") });
+router.post(
+  "/admin/invitations",
+  asyncHandler(async (req, res) => {
+    const body = parseBody(inviteSchema, req.body);
+    const admin = await prisma.user.findUnique({ where: { id: req.userId! } });
+    if (!admin) throw new HttpError(401, "Invalid or expired token");
+    const invitation = await inviteUser({
+      email: body.email,
+      invitedById: admin.id,
+      inviterName: admin.displayName,
+      origin: req.get("origin") ?? null,
+    });
+    void createLog({ userId: admin.id, action: "user_invited", details: { email: invitation.email } });
+    res.json({ email: invitation.email, expires_at: invitation.expiresAt, expires_in_days: INVITATION_TTL_DAYS });
+  }),
+);
+
 // The two-part "select a user + type their email to confirm" flow lives entirely on the
 // frontend (TriggersSection.tsx) -- this endpoint just does the (irreversible) deletion once
 // asked, trusting the UI already got explicit confirmation from the admin.
diff --git a/backend/src/routes/auth.ts b/backend/src/routes/auth.ts
index 83da311..d6d6ec1 100644
--- a/backend/src/routes/auth.ts
+++ b/backend/src/routes/auth.ts
@@ -9,6 +9,7 @@ import { parseBody } from "../utils/validate";
 import { asyncHandler } from "../utils/asyncHandler";
 import { getAllowRegistrations, isSmtpConfigured } from "../services/settingsService";
 import { sendVerificationEmail } from "../services/mailer";
+import { findValidInvitation } from "../services/invitationService";
 import { createLog } from "../services/auditLog";
 import { seedDefaultCategories } from "../services/categoryService";
 import { toUserOut } from "../dto";
@@ -27,6 +28,9 @@ const registerSchema = z.object({
   displayName: z.string().trim().min(1, "Display name is required"),
   email: z.string().trim().email("Enter a valid email address"),
   password: z.string().min(8, "Password must be at least 8 characters"),
+  // From an invitation link (services/invitationService.ts) -- the only way to register while
+  // registrations are closed.
+  invite_token: z.string().min(1).optional(),
 });
 
 const loginSchema = z.object({
@@ -46,7 +50,17 @@ router.post(
     const adminExists = (await prisma.user.count({ where: { role: "ADMIN" } })) > 0;
     const bootstrapping = !adminExists;
 
-    if (!bootstrapping && !(await getAllowRegistrations(true))) {
+    // An invitation lets its own email in even while registrations are closed, and only that
+    // email: the token has to match an unexpired invitation *for the address being registered*.
+    const invitation = body.invite_token ? await findValidInvitation(body.invite_token) : null;
+    if (body.invite_token && !invitation) {
+      throw new HttpError(400, "This invitation link is invalid or has expired. Ask for a new one.");
+    }
+    if (invitation && invitation.email !== email) {
+      throw new HttpError(400, "This invitation is for a different email address.");
+    }
+
+    if (!bootstrapping && !invitation && !(await getAllowRegistrations(true))) {
       throw new HttpError(403, "Registration is currently disabled");
     }
     if (await prisma.user.findUnique({ where: { email } })) {
@@ -60,7 +74,9 @@ router.post(
     // Otherwise, email verification only actually happens when SMTP is configured; unconfigured
     // instances create fully-verified accounts immediately so this feature is opt-in, not a
     // requirement that breaks self-hosters who never set up a mail server.
-    const smtpConfigured = !bootstrapping && (await isSmtpConfigured());
+    // An invited user skips it too: they reached this form through a link sent to that very
+    // address, which already proves they own the mailbox.
+    const smtpConfigured = !bootstrapping && !invitation && (await isSmtpConfigured());
     const verification = smtpConfigured ? newVerificationToken() : null;
 
     // Wrapped together so an account never ends up missing its starter categories (or vice
@@ -78,6 +94,8 @@ router.post(
         },
       });
       await seedDefaultCategories(tx, created.id);
+      // Single use: the invitation goes away with the account it created.
+      if (invitation) await tx.invitation.delete({ where: { id: invitation.id } });
       return created;
     }, { timeout: 15000 }); // seedDefaultCategories is ~80 sequential inserts -- Prisma's 5s default is too tight
 
@@ -120,6 +138,17 @@ router.post(
   }),
 );
 
+// Backs the invitation link's registration form: confirms the link is still good before the
+// invitee fills anything in. Returns only the invited email, which the link already contains.
+router.get(
+  "/invitations/:token",
+  asyncHandler(async (req, res) => {
+    const invitation = await findValidInvitation(req.params.token);
+    if (!invitation) throw new HttpError(404, "This invitation link is invalid or has expired. Ask for a new one.");
+    res.json({ email: invitation.email, expires_at: invitation.expiresAt });
+  }),
+);
+
 const verifyEmailSchema = z.object({ token: z.string().min(1) });
 
 router.post(
diff --git a/backend/src/routes/health.ts b/backend/src/routes/health.ts
index 64ba433..5438958 100644
--- a/backend/src/routes/health.ts
+++ b/backend/src/routes/health.ts
@@ -1,5 +1,6 @@
 import { Router } from "express";
 import { asyncHandler } from "../utils/asyncHandler";
+import { prisma } from "../db";
 import { getAllowRegistrations } from "../services/settingsService";
 
 const router = Router();
@@ -7,7 +8,10 @@ const router = Router();
 router.get(
   "/health",
   asyncHandler(async (_req, res) => {
-    res.json({ ok: true, auth_required: true, allow_registrations: await getAllowRegistrations(true) });
+    // Whether the sign-in page offers a Register tab. Always true until the first admin exists --
+    // that first account can register whatever the setting says (see routes/auth.ts).
+    const bootstrapping = (await prisma.user.count({ where: { role: "ADMIN" } })) === 0;
+    res.json({ ok: true, auth_required: true, allow_registrations: bootstrapping || (await getAllowRegistrations(true)) });
   }),
 );
 
diff --git a/backend/src/routes/settings.ts b/backend/src/routes/settings.ts
index 521e7f2..af1c462 100644
--- a/backend/src/routes/settings.ts
+++ b/backend/src/routes/settings.ts
@@ -95,8 +95,8 @@ router.get(
   }),
 );
 
-// No admin UI calls this yet -- added now so the "admin panel later" plan has a working
-// endpoint to build against without another backend change.
+// Administration > Users. Closing registrations makes POST /register refuse everyone but
+// invitees (see services/invitationService.ts) and the very first account.
 const registrationsSchema = z.object({ allow_registrations: z.boolean() });
 router.post(
   "/settings/registrations",
diff --git a/backend/src/services/auditLog.ts b/backend/src/services/auditLog.ts
index ce65744..afb3586 100644
--- a/backend/src/services/auditLog.ts
+++ b/backend/src/services/auditLog.ts
@@ -4,6 +4,7 @@ import { prisma } from "../db";
 export type LogAction =
   | "user_logged_in"
   | "user_logged_out"
+  | "user_invited"
   | "model_uploaded"
   | "model_imported"
   | "import_completed"
diff --git a/backend/src/services/invitationService.ts b/backend/src/services/invitationService.ts
new file mode 100644
index 0000000..1b0d09b
--- /dev/null
+++ b/backend/src/services/invitationService.ts
@@ -0,0 +1,74 @@
+// Invitations to register while new registrations are turned off (Administration > Users).
+// An admin enters an email; the invitee gets a link with a single-use token, and registering with
+// that token is the only way to create an account for that address (see routes/auth.ts). The token
+// -- not just the email in the link -- is what authorizes it: otherwise anyone who knew an invited
+// address could register it first and lock the real invitee out. It also proves the invitee owns
+// the mailbox, so accounts created from an invitation skip the separate verification email.
+
+import crypto from "node:crypto";
+import type { Invitation } from "@prisma/client";
+import { PUBLIC_URL } from "../config";
+import { prisma } from "../db";
+import { HttpError } from "../utils/fileUtils";
+import { sendInvitationEmail } from "./mailer";
+import { getAllowRegistrations, isSmtpConfigured } from "./settingsService";
+
+export const INVITATION_TTL_DAYS = 7;
+
+/** The registration link an invitation email points to. `origin` (the admin's own browser origin)
+ *  is only a fallback for instances without PUBLIC_URL -- it's the address the admin reached the
+ *  frontend at, which is exactly what the invitee needs too. */
+export function invitationLink(email: string, token: string, origin: string | null): string {
+  const base = PUBLIC_URL || (origin ?? "").replace(/\/+$/, "");
+  return `${base}/register?email=${encodeURIComponent(email)}&invite=${encodeURIComponent(token)}`;
+}
+
+/** Creates (or, for an address already invited, replaces) the invitation and emails it. Only
+ *  allowed while registrations are closed -- open registrations need no invitation -- and with SMTP
+ *  set up, since the link only ever travels by email. */
+export async function inviteUser(params: { email: string; invitedById: string; inviterName: string; origin: string | null }): Promise<Invitation> {
+  const email = params.email.trim().toLowerCase();
+  if (await getAllowRegistrations(true)) {
+    throw new HttpError(400, "Registrations are open, so anyone can sign up without an invitation.");
+  }
+  if (!(await isSmtpConfigured())) {
+    throw new HttpError(400, "Set up SMTP (Connections > SMTP) to send invitations.");
+  }
+  if (await prisma.user.findUnique({ where: { email } })) {
+    throw new HttpError(409, "An account with this email already exists");
+  }
+
+  const token = crypto.randomBytes(32).toString("hex");
+  const expiresAt = new Date(Date.now() + INVITATION_TTL_DAYS * 24 * 60 * 60 * 1000);
+  const previous = await prisma.invitation.findUnique({ where: { email } });
+  const invitation = await prisma.invitation.upsert({
+    where: { email },
+    create: { email, token, expiresAt, invitedById: params.invitedById },
+    update: { token, expiresAt, invitedById: params.invitedById },
+  });
+
+  try {
+    await sendInvitationEmail(email, params.inviterName, invitationLink(email, token, params.origin), INVITATION_TTL_DAYS);
+  } catch (err) {
+    console.error("[invitations] Failed to send invitation email:", err);
+    // Leave things as they were: no brand-new invitation nobody received, and a re-invite keeps
+    // the earlier (already delivered) link working.
+    if (previous) {
+      await prisma.invitation.update({
+        where: { email },
+        data: { token: previous.token, expiresAt: previous.expiresAt, invitedById: previous.invitedById },
+      });
+    } else {
+      await prisma.invitation.delete({ where: { email } }).catch(() => undefined);
+    }
+    throw new HttpError(500, "Failed to send the invitation email. Check the SMTP settings and try again.");
+  }
+  return invitation;
+}
+
+/** The invitation behind a registration link, or null if the token is unknown or expired. */
+export async function findValidInvitation(token: string): Promise<Invitation | null> {
+  const invitation = await prisma.invitation.findUnique({ where: { token } });
+  if (!invitation || invitation.expiresAt < new Date()) return null;
+  return invitation;
+}
diff --git a/backend/src/services/mailer.ts b/backend/src/services/mailer.ts
index fa63aea..4a62a55 100644
--- a/backend/src/services/mailer.ts
+++ b/backend/src/services/mailer.ts
@@ -2,28 +2,46 @@ import nodemailer from "nodemailer";
 import { PUBLIC_URL } from "../config";
 import { getSmtpSettings } from "./settingsService";
 
-/** Sends the account-verification email a new registration (see routes/auth.ts) waits on before
- * it can sign in. Only ever called once the caller has already confirmed SMTP is configured
- * (isSmtpConfigured) -- throws if `host` somehow ends up unset here anyway, since silently
- * dropping a verification email would leave the account permanently unreachable. */
-export async function sendVerificationEmail(to: string, displayName: string, token: string): Promise<void> {
+/** Sends one email through the instance's SMTP settings. Only ever called once the caller has
+ * already confirmed SMTP is configured (isSmtpConfigured) -- throws if `host` somehow ends up
+ * unset here anyway, since silently dropping one of these emails would leave its recipient with
+ * no way in. */
+async function sendMail(message: { to: string; subject: string; text: string; html: string }): Promise<void> {
   const smtp = await getSmtpSettings();
   if (!smtp.host) throw new Error("SMTP is not configured");
 
-  const link = `${PUBLIC_URL}/verify-email?token=${encodeURIComponent(token)}`;
-
   const transporter = nodemailer.createTransport({
     host: smtp.host,
     port: smtp.port,
     secure: smtp.secure,
     auth: smtp.user ? { user: smtp.user, pass: smtp.pass ?? undefined } : undefined,
   });
+  await transporter.sendMail({ from: smtp.from, ...message });
+}
 
-  await transporter.sendMail({
-    from: smtp.from,
+function escapeHtml(value: string): string {
+  return value.replace(/[&<>"']/g, (c) => ({ "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;", "'": "&#39;" })[c]!);
+}
+
+/** Sends the account-verification email a new registration (see routes/auth.ts) waits on before
+ * it can sign in. */
+export async function sendVerificationEmail(to: string, displayName: string, token: string): Promise<void> {
+  const link = `${PUBLIC_URL}/verify-email?token=${encodeURIComponent(token)}`;
+  await sendMail({
     to,
     subject: "Confirm your Thingport account",
     text: `Hi ${displayName},\n\nConfirm your email address to finish creating your Thingport account:\n${link}\n\nThis link expires in 24 hours.`,
-    html: `<p>Hi ${displayName},</p><p>Confirm your email address to finish creating your Thingport account:</p><p><a href="${link}">${link}</a></p><p>This link expires in 24 hours.</p>`,
+    html: `<p>Hi ${escapeHtml(displayName)},</p><p>Confirm your email address to finish creating your Thingport account:</p><p><a href="${link}">${link}</a></p><p>This link expires in 24 hours.</p>`,
+  });
+}
+
+/** Sends an invitation to register while registrations are closed (see
+ * services/invitationService.ts). `link` is the full registration URL, token included. */
+export async function sendInvitationEmail(to: string, inviterName: string, link: string, expiresInDays: number): Promise<void> {
+  await sendMail({
+    to,
+    subject: "You're invited to Thingport",
+    text: `${inviterName} invited you to create an account on their Thingport, a library for 3D-printing models.\n\nCreate your account here:\n${link}\n\nThe link works for ${expiresInDays} days, for this email address only.`,
+    html: `<p>${escapeHtml(inviterName)} invited you to create an account on their Thingport, a library for 3D-printing models.</p><p><a href="${link}">Create your account</a></p><p>Or open this link: ${link}</p><p>The link works for ${expiresInDays} days, for this email address only.</p>`,
   });
 }
diff --git a/backend/tests/invitations.test.ts b/backend/tests/invitations.test.ts
new file mode 100644
index 0000000..7c8b193
--- /dev/null
+++ b/backend/tests/invitations.test.ts
@@ -0,0 +1,228 @@
+import { afterAll, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+import request from "supertest";
+
+// No real email goes anywhere: the mailer is replaced, and each test reads the invitation link it
+// was asked to send from the mock's calls.
+vi.mock("../src/services/mailer", () => ({
+  sendVerificationEmail: vi.fn<(to: string, displayName: string, token: string) => Promise<void>>(async () => undefined),
+  sendInvitationEmail: vi.fn<(to: string, inviterName: string, link: string, expiresInDays: number) => Promise<void>>(
+    async () => undefined,
+  ),
+}));
+
+import { createApp } from "../src/app";
+import { prisma } from "../src/db";
+import { sendInvitationEmail, sendVerificationEmail } from "../src/services/mailer";
+import {
+  getAllowRegistrations,
+  getSmtpSettings,
+  setAllowRegistrations,
+  setSmtpSettings,
+} from "../src/services/settingsService";
+
+const app = createApp();
+const sendInvitation = vi.mocked(sendInvitationEmail);
+const sendVerification = vi.mocked(sendVerificationEmail);
+let adminToken: string;
+let memberToken: string;
+// Every test file shares one database (see vitest.config.ts), so the instance-wide settings this
+// file changes are put back afterwards.
+let previousSmtpHost: string | null;
+let previousAllowRegistrations: boolean;
+
+const stamp = Date.now();
+const email = (name: string) => `${name}-${stamp}@example.com`;
+const auth = (token: string) => ({ Authorization: `Bearer ${token}` });
+
+/** The token from the link in the most recent invitation email. */
+function lastInvitationToken(): string {
+  const link = sendInvitation.mock.calls.at(-1)![2];
+  return new URL(link, "http://thingport.test").searchParams.get("invite")!;
+}
+
+async function closeRegistrationsWithSmtp() {
+  await setAllowRegistrations(false);
+  await setSmtpSettings({ host: "smtp.example.test" });
+}
+
+beforeAll(async () => {
+  previousSmtpHost = (await getSmtpSettings()).host;
+  previousAllowRegistrations = await getAllowRegistrations(true);
+  await setAllowRegistrations(true);
+  await setSmtpSettings({ host: null });
+
+  const adminRegister = await request(app)
+    .post("/api/register")
+    .send({ displayName: "Invite Admin", email: email("invite-admin"), password: "password123" });
+  await prisma.user.update({ where: { id: adminRegister.body.user.id }, data: { role: "ADMIN" } });
+  adminToken = (await request(app).post("/api/login").send({ email: email("invite-admin"), password: "password123" })).body.token;
+
+  const memberRegister = await request(app)
+    .post("/api/register")
+    .send({ displayName: "Invite Member", email: email("invite-member"), password: "password123" });
+  memberToken = memberRegister.body.token;
+});
+
+afterAll(async () => {
+  await setAllowRegistrations(previousAllowRegistrations);
+  await setSmtpSettings({ host: previousSmtpHost });
+});
+
+beforeEach(async () => {
+  sendInvitation.mockClear();
+  sendVerification.mockClear();
+  await setAllowRegistrations(true);
+  await setSmtpSettings({ host: null });
+});
+
+describe("closing registrations", () => {
+  it("refuses a plain registration and hides registration from the sign-in page", async () => {
+    await setAllowRegistrations(false);
+    const res = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Nobody", email: email("closed"), password: "password123" });
+    expect(res.status).toBe(403);
+    expect((await request(app).get("/api/health")).body.allow_registrations).toBe(false);
+  });
+
+  it("only lets an admin change the setting", async () => {
+    const asMember = await request(app).post("/api/settings/registrations").set(auth(memberToken)).send({ allow_registrations: false });
+    expect(asMember.status).toBe(403);
+    const asAdmin = await request(app).post("/api/settings/registrations").set(auth(adminToken)).send({ allow_registrations: false });
+    expect(asAdmin.status).toBe(200);
+    expect(await getAllowRegistrations(true)).toBe(false);
+  });
+});
+
+describe("inviting", () => {
+  it("is admin-only", async () => {
+    await closeRegistrationsWithSmtp();
+    const res = await request(app).post("/api/admin/invitations").set(auth(memberToken)).send({ email: email("by-member") });
+    expect(res.status).toBe(403);
+    expect(sendInvitation).not.toHaveBeenCalled();
+  });
+
+  it("refuses while registrations are open", async () => {
+    await setSmtpSettings({ host: "smtp.example.test" });
+    const res = await request(app).post("/api/admin/invitations").set(auth(adminToken)).send({ email: email("open") });
+    expect(res.status).toBe(400);
+    expect(sendInvitation).not.toHaveBeenCalled();
+  });
+
+  it("refuses without SMTP", async () => {
+    await setAllowRegistrations(false);
+    const res = await request(app).post("/api/admin/invitations").set(auth(adminToken)).send({ email: email("no-smtp") });
+    expect(res.status).toBe(400);
+    expect(sendInvitation).not.toHaveBeenCalled();
+  });
+
+  it("refuses an address that already has an account", async () => {
+    await closeRegistrationsWithSmtp();
+    const res = await request(app).post("/api/admin/invitations").set(auth(adminToken)).send({ email: email("invite-member") });
+    expect(res.status).toBe(409);
+  });
+
+  it("emails a registration link carrying the email and a token, and logs it", async () => {
+    await closeRegistrationsWithSmtp();
+    const invitee = email("Invitee-Case");
+    const res = await request(app)
+      .post("/api/admin/invitations")
+      .set({ ...auth(adminToken), Origin: "https://thingport.example.test" })
+      .send({ email: invitee });
+    expect(res.status).toBe(200);
+    expect(res.body.email).toBe(invitee.toLowerCase());
+
+    expect(sendInvitation).toHaveBeenCalledTimes(1);
+    const [to, inviterName, link] = sendInvitation.mock.calls[0];
+    expect(to).toBe(invitee.toLowerCase());
+    expect(inviterName).toBe("Invite Admin");
+    const url = new URL(link);
+    expect(url.pathname).toBe("/register");
+    expect(url.searchParams.get("email")).toBe(invitee.toLowerCase());
+    expect(url.searchParams.get("invite")).toMatch(/^[0-9a-f]{64}$/);
+
+    const lookup = await request(app).get(`/api/invitations/${lastInvitationToken()}`);
+    expect(lookup.status).toBe(200);
+    expect(lookup.body.email).toBe(invitee.toLowerCase());
+
+    const log = await prisma.log.findFirst({ where: { action: "user_invited" }, orderBy: { createdAt: "desc" } });
+    expect(log?.details).toMatchObject({ email: invitee.toLowerCase() });
+  });
+
+  it("leaves no invitation behind when the email can't be sent", async () => {
+    await closeRegistrationsWithSmtp();
+    sendInvitation.mockRejectedValueOnce(new Error("SMTP down"));
+    const invitee = email("send-fails");
+    const res = await request(app).post("/api/admin/invitations").set(auth(adminToken)).send({ email: invitee });
+    expect(res.status).toBe(500);
+    expect(await prisma.invitation.findUnique({ where: { email: invitee } })).toBeNull();
+  });
+});
+
+describe("registering with an invitation", () => {
+  async function invite(address: string): Promise<string> {
+    await closeRegistrationsWithSmtp();
+    const res = await request(app).post("/api/admin/invitations").set(auth(adminToken)).send({ email: address });
+    expect(res.status).toBe(200);
+    return lastInvitationToken();
+  }
+
+  it("creates a verified account while registrations are closed, and the link works only once", async () => {
+    const invitee = email("accepts");
+    const token = await invite(invitee);
+
+    const res = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Invited User", email: invitee, password: "password123", invite_token: token });
+    expect(res.status).toBe(200);
+    // Signed straight in: the invitation link already proved the mailbox, so no verification email.
+    expect(res.body.token).toBeTruthy();
+    expect(res.body.user.email).toBe(invitee);
+    expect(sendVerification).not.toHaveBeenCalled();
+    expect(await prisma.invitation.findUnique({ where: { email: invitee } })).toBeNull();
+
+    const again = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Second Try", email: email("reuse"), password: "password123", invite_token: token });
+    expect(again.status).toBe(400);
+    expect((await request(app).get(`/api/invitations/${token}`)).status).toBe(404);
+  });
+
+  it("refuses the token for a different email", async () => {
+    const token = await invite(email("owner"));
+    const res = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Someone Else", email: email("not-owner"), password: "password123", invite_token: token });
+    expect(res.status).toBe(400);
+    expect(await prisma.user.findUnique({ where: { email: email("not-owner") } })).toBeNull();
+  });
+
+  it("refuses an unknown token", async () => {
+    await closeRegistrationsWithSmtp();
+    const res = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Guesser", email: email("guesser"), password: "password123", invite_token: "0".repeat(64) });
+    expect(res.status).toBe(400);
+  });
+
+  it("retires the old link when an address is invited again", async () => {
+    const invitee = email("reinvited");
+    const firstToken = await invite(invitee);
+    const secondToken = await invite(invitee);
+    expect(secondToken).not.toBe(firstToken);
+    expect((await request(app).get(`/api/invitations/${firstToken}`)).status).toBe(404);
+    expect((await request(app).get(`/api/invitations/${secondToken}`)).status).toBe(200);
+  });
+
+  it("refuses an expired invitation", async () => {
+    const invitee = email("expired");
+    const token = await invite(invitee);
+    await prisma.invitation.update({ where: { email: invitee }, data: { expiresAt: new Date(Date.now() - 1000) } });
+
+    expect((await request(app).get(`/api/invitations/${token}`)).status).toBe(404);
+    const res = await request(app)
+      .post("/api/register")
+      .send({ displayName: "Too Late", email: invitee, password: "password123", invite_token: token });
+    expect(res.status).toBe(400);
+  });
+});
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 0827570..cf8df50 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -373,6 +373,15 @@ export default function App() {
     // pre-login from the link in the verification email -- is checked directly against
     // window.location rather than via a route.
     const isVerifyEmailPath = typeof window !== "undefined" && window.location.pathname === "/verify-email";
+    // Same for /register?email=...&invite=... -- the link in an invitation email (see the backend's
+    // services/invitationService.ts), which opens the register form for that one address even
+    // while registrations are closed.
+    const inviteParams = typeof window !== "undefined" && window.location.pathname === "/register"
+      ? new URLSearchParams(window.location.search)
+      : null;
+    const invite = inviteParams?.get("invite")
+      ? { token: inviteParams.get("invite")!, email: inviteParams.get("email") ?? "" }
+      : null;
     return (
       <ThemeProvider theme={muiTheme}>
         <CssBaseline />
@@ -393,6 +402,7 @@ export default function App() {
               onSuccess={handleLogin}
               apiUp={apiUp}
               allowRegistrations={health?.allow_registrations ?? true}
+              invite={invite}
             />
           )}
         </Box>
diff --git a/frontend/src/api/admin.ts b/frontend/src/api/admin.ts
index 70f92bb..af2a107 100644
--- a/frontend/src/api/admin.ts
+++ b/frontend/src/api/admin.ts
@@ -15,6 +15,7 @@ export type AdminUser = {
 export type LogAction =
   | "user_logged_in"
   | "user_logged_out"
+  | "user_invited"
   | "model_uploaded"
   | "model_imported"
   | "import_completed"
@@ -49,6 +50,17 @@ export const adminApi = {
     return res.json();
   },
 
+  /** Emails an invitation to register (only while registrations are closed and SMTP is set up). */
+  inviteUser: async (email: string): Promise<{ email: string; expires_at: string; expires_in_days: number }> => {
+    const res = await fetch(`${apiBase()}/admin/invitations`, {
+      method: "POST",
+      headers: authHeaders({ "Content-Type": "application/json" }),
+      body: JSON.stringify({ email }),
+    });
+    if (!res.ok) throw new Error(await readErrorMessage(res, "Failed to send the invitation"));
+    return res.json();
+  },
+
   listUsers: async (): Promise<AdminUser[]> => {
     const res = await fetch(`${apiBase()}/admin/users`, { headers: authHeaders() });
     assertOk(res, "Failed to load users");
diff --git a/frontend/src/api/auth.ts b/frontend/src/api/auth.ts
index ba69d95..0d03c08 100644
--- a/frontend/src/api/auth.ts
+++ b/frontend/src/api/auth.ts
@@ -51,8 +51,21 @@ async function postAuth<T>(path: string, body: unknown): Promise<T> {
 export const authApi = {
   login: (email: string, password: string): Promise<AuthResult> => postAuth("/login", { email, password }),
 
-  register: (payload: { displayName: string; email: string; password: string }): Promise<RegisterResult> =>
-    postAuth("/register", { displayName: payload.displayName, email: payload.email, password: payload.password }),
+  // `inviteToken` comes from an invitation link -- the only way in while registrations are closed.
+  register: (payload: { displayName: string; email: string; password: string; inviteToken?: string }): Promise<RegisterResult> =>
+    postAuth("/register", {
+      displayName: payload.displayName,
+      email: payload.email,
+      password: payload.password,
+      ...(payload.inviteToken ? { invite_token: payload.inviteToken } : {}),
+    }),
+
+  /** Checks an invitation link before its form is filled in -- rejects if it's invalid or expired. */
+  getInvitation: async (token: string): Promise<{ email: string; expires_at: string }> => {
+    const res = await fetch(`${apiBase()}/invitations/${encodeURIComponent(token)}`);
+    if (!res.ok) return readAuthError(res);
+    return res.json();
+  },
 
   verifyEmail: (token: string): Promise<AuthResult> => postAuth("/verify-email", { token }),
 
diff --git a/frontend/src/api/settings.ts b/frontend/src/api/settings.ts
index 608eede..20e9f39 100644
--- a/frontend/src/api/settings.ts
+++ b/frontend/src/api/settings.ts
@@ -114,6 +114,27 @@ export const settingsApi = {
     return res.json();
   },
 
+  // Instance-wide: whether anyone may register. Closed means only invited emails (adminApi.inviteUser)
+  // -- and the very first account -- can.
+  getRegistrations: async (): Promise<{ allow_registrations: boolean }> => {
+    const res = await fetch(`${apiBase()}/settings/registrations`, { headers: authHeaders() });
+    assertOk(res, "Failed to load registration settings");
+    return res.json();
+  },
+
+  updateRegistrations: async (allowRegistrations: boolean): Promise<{ allow_registrations: boolean }> => {
+    const res = await fetch(`${apiBase()}/settings/registrations`, {
+      method: "POST",
+      headers: authHeaders({ "Content-Type": "application/json" }),
+      body: JSON.stringify({ allow_registrations: allowRegistrations }),
+    });
+    if (res.status === 401) throw new UnauthorizedError();
+    if (!res.ok) {
+      throw new Error(await readErrorMessage(res, "Failed to update registration settings"));
+    }
+    return res.json();
+  },
+
   // Instance-wide Thingiverse Developer API Access Token, shared by every user's Thingiverse
   // imports -- write-only like any other API secret: GET only ever reports whether one is
   // configured, never the value itself.
diff --git a/frontend/src/i18n/locales/en.json b/frontend/src/i18n/locales/en.json
index 19dc7dc..6bbb551 100644
--- a/frontend/src/i18n/locales/en.json
+++ b/frontend/src/i18n/locales/en.json
@@ -29,7 +29,9 @@
         "failed": "Registration failed",
         "passwordMismatch": "Passwords do not match",
         "passwordTooShort": "Password must be at least 8 characters",
-        "disabled": "New registrations are currently disabled."
+        "invited": "You've been invited to Thingport. Pick a display name and password to create your account.",
+        "inviteEmailLocked": "Your invitation is for this address.",
+        "inviteInvalid": "This invitation link is invalid or has expired. Ask whoever invited you for a new one."
       },
       "checkEmail": {
         "heading": "Check your email",
@@ -297,6 +299,25 @@
         "savedNoApply": "Storage structure saved for new and updated models.",
         "failed": "Failed to save storage settings."
       },
+      "registrations": {
+        "allowLabel": "Allow new registrations",
+        "openHelp": "Anyone who can reach this instance can create an account from the sign-in page.",
+        "closedHelp": "The sign-in page has no Register option, and new accounts can only be created from an invitation.",
+        "inviteButton": "Invite users",
+        "inviteHelp": "Sends a registration link by email. It works for one address, once, for 7 days.",
+        "failed": "Failed to update registration settings",
+        "invite": {
+          "title": "Invite a user",
+          "description": "They'll get an email with a link to create their account. Only this address can use it.",
+          "emailLabel": "Email address",
+          "send": "Send invitation",
+          "sending": "Sending...",
+          "close": "Close",
+          "sent_one": "Invitation sent to {{email}}. The link works for {{count}} day.",
+          "sent_other": "Invitation sent to {{email}}. The link works for {{count}} days.",
+          "failed": "Failed to send the invitation"
+        }
+      },
       "previews": {
         "heading": "Model Previews",
         "subtitle": "Control background preview generation for the whole instance.",
@@ -446,6 +467,7 @@
         "action": {
           "user_logged_in": "Logged in",
           "user_logged_out": "Logged out",
+          "user_invited": "Invited a user",
           "model_uploaded": "Model uploaded",
           "model_imported": "Model imported",
           "import_completed": "Import completed",
diff --git a/frontend/src/i18n/locales/lt.json b/frontend/src/i18n/locales/lt.json
index b042ab5..a5ab985 100644
--- a/frontend/src/i18n/locales/lt.json
+++ b/frontend/src/i18n/locales/lt.json
@@ -29,7 +29,9 @@
         "failed": "Registracija nepavyko",
         "passwordMismatch": "Slaptažodžiai nesutampa",
         "passwordTooShort": "Slaptažodis turi būti bent 8 simbolių",
-        "disabled": "Šiuo metu naujos registracijos yra išjungtos."
+        "invited": "Jūs pakviesti į Thingport. Pasirinkite rodomą vardą ir slaptažodį, kad sukurtumėte paskyrą.",
+        "inviteEmailLocked": "Kvietimas skirtas šiam adresui.",
+        "inviteInvalid": "Ši kvietimo nuoroda negalioja arba jos galiojimas baigėsi. Paprašykite jus pakvietusio žmogaus naujos."
       },
       "checkEmail": {
         "heading": "Patikrinkite savo el. paštą",
@@ -298,6 +300,26 @@
         "savedNoApply": "Saugojimo struktūra išsaugota naujiems ir atnaujintiems modeliams.",
         "failed": "Nepavyko išsaugoti saugojimo nustatymų."
       },
+      "registrations": {
+        "allowLabel": "Leisti naujas registracijas",
+        "openHelp": "Bet kas, pasiekiantis šį serverį, gali susikurti paskyrą prisijungimo puslapyje.",
+        "closedHelp": "Prisijungimo puslapyje nėra registracijos, o naujas paskyras galima sukurti tik gavus kvietimą.",
+        "inviteButton": "Kviesti naudotojus",
+        "inviteHelp": "El. paštu išsiunčia registracijos nuorodą. Ją vieną kartą gali panaudoti tik vienas adresas, 7 dienas.",
+        "failed": "Nepavyko atnaujinti registracijos nustatymų",
+        "invite": {
+          "title": "Pakviesti naudotoją",
+          "description": "Gavėjas gaus el. laišką su nuoroda paskyrai sukurti. Ja gali pasinaudoti tik šis adresas.",
+          "emailLabel": "El. pašto adresas",
+          "send": "Siųsti kvietimą",
+          "sending": "Siunčiama...",
+          "close": "Uždaryti",
+          "sent_one": "Kvietimas išsiųstas adresu {{email}}. Nuoroda galios {{count}} dieną.",
+          "sent_few": "Kvietimas išsiųstas adresu {{email}}. Nuoroda galios {{count}} dienas.",
+          "sent_other": "Kvietimas išsiųstas adresu {{email}}. Nuoroda galios {{count}} dienų.",
+          "failed": "Nepavyko išsiųsti kvietimo"
+        }
+      },
       "previews": {
         "heading": "Modelių peržiūros",
         "subtitle": "Valdykite fono peržiūrų generavimą visai sistemai.",
@@ -447,6 +469,7 @@
         "action": {
           "user_logged_in": "Prisijungė",
           "user_logged_out": "Atsijungė",
+          "user_invited": "Pakvietė naudotoją",
           "model_uploaded": "Modelis įkeltas",
           "model_imported": "Modelis importuotas",
           "import_completed": "Importas baigtas",
diff --git a/frontend/src/pages/AuthPage/RegisterPanel.tsx b/frontend/src/pages/AuthPage/RegisterPanel.tsx
index a2b0957..13ce6ab 100644
--- a/frontend/src/pages/AuthPage/RegisterPanel.tsx
+++ b/frontend/src/pages/AuthPage/RegisterPanel.tsx
@@ -5,21 +5,29 @@ import Stack from "@mui/material/Stack";
 import TextField from "@mui/material/TextField";
 import Button from "@mui/material/Button";
 import Alert from "@mui/material/Alert";
+import CircularProgress from "@mui/material/CircularProgress";
 import Typography from "@mui/material/Typography";
 import { authApi, type AuthUser } from "../../api/auth";
 import CheckEmailPanel from "./CheckEmailPanel";
 
 const MIN_PASSWORD_LENGTH = 8;
 
+/** From an invitation link's query string (see App.tsx). */
+export type Invite = { token: string; email: string };
+
 type Props = {
   onSuccess: (token: string, expires_in: number, user: AuthUser) => void;
-  allowRegistrations: boolean;
+  invite?: Invite | null;
 };
 
-export default function RegisterPanel({ onSuccess, allowRegistrations }: Props) {
+export default function RegisterPanel({ onSuccess, invite = null }: Props) {
   const { t } = useTranslation("app");
   const [displayName, setDisplayName] = React.useState("");
-  const [email, setEmail] = React.useState("");
+  const [email, setEmail] = React.useState(invite?.email ?? "");
+  // An invitation is checked with the backend before the form opens: "checking" until then,
+  // "valid" (email locked to the invited address) or "invalid" (form stays closed).
+  const [inviteState, setInviteState] = React.useState<"checking" | "valid" | "invalid">(invite ? "checking" : "valid");
+  const [inviteError, setInviteError] = React.useState<string | null>(null);
   const [password, setPassword] = React.useState("");
   const [confirmPassword, setConfirmPassword] = React.useState("");
   const [loading, setLoading] = React.useState(false);
@@ -28,6 +36,25 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
   // form with CheckEmailPanel instead of ever calling onSuccess.
   const [pendingEmail, setPendingEmail] = React.useState<string | null>(null);
 
+  // Keyed on the token string, not the invite object -- App.tsx builds that afresh every render.
+  const inviteToken = invite?.token ?? null;
+  React.useEffect(() => {
+    if (!inviteToken) return;
+    let active = true;
+    authApi.getInvitation(inviteToken)
+      .then(res => {
+        if (!active) return;
+        setEmail(res.email);
+        setInviteState("valid");
+      })
+      .catch(err => {
+        if (!active) return;
+        setInviteError(err instanceof Error ? err.message : t("auth.register.inviteInvalid"));
+        setInviteState("invalid");
+      });
+    return () => { active = false; };
+  }, [inviteToken, t]);
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     setError(null);
@@ -41,10 +68,13 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
     }
     setLoading(true);
     try {
-      const res = await authApi.register({ displayName, email, password });
+      const res = await authApi.register({ displayName, email, password, inviteToken: invite?.token });
       if ("email_verification_required" in res) {
         setPendingEmail(res.email);
       } else {
+        // Leave /register?invite=... behind: the signed-in app doesn't have that route, and the
+        // one-time link shouldn't linger in the address bar or history.
+        if (invite) window.history.replaceState(null, "", "/");
         onSuccess(res.token, res.expires_in, res.user);
       }
     } catch (err) {
@@ -59,10 +89,22 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
     return <CheckEmailPanel email={pendingEmail} />;
   }
 
+  if (inviteState === "checking") {
+    return (
+      <Box sx={{ display: "flex", justifyContent: "center", py: 4 }}>
+        <CircularProgress size={28} />
+      </Box>
+    );
+  }
+
+  if (inviteState === "invalid") {
+    return <Alert severity="error">{inviteError ?? t("auth.register.inviteInvalid")}</Alert>;
+  }
+
   return (
     <Box component="form" onSubmit={handleSubmit}>
       <Stack spacing={2}>
-        {!allowRegistrations && <Alert severity="info">{t("auth.register.disabled")}</Alert>}
+        {invite && <Alert severity="info">{t("auth.register.invited")}</Alert>}
         {error && <Alert severity="error">{error}</Alert>}
         <TextField
           label={t("auth.register.displayNameLabel")}
@@ -72,7 +114,6 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
           required
           fullWidth
           size="small"
-          disabled={!allowRegistrations}
         />
         <TextField
           type="email"
@@ -83,7 +124,8 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
           required
           fullWidth
           size="small"
-          disabled={!allowRegistrations}
+          disabled={invite !== null}
+          helperText={invite ? t("auth.register.inviteEmailLocked") : undefined}
         />
         <Stack spacing={0.5}>
           <TextField
@@ -95,7 +137,6 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
             required
             fullWidth
             size="small"
-            disabled={!allowRegistrations}
           />
           <Typography variant="caption" color="text.secondary">
             {t("auth.register.passwordHelp")}
@@ -110,9 +151,8 @@ export default function RegisterPanel({ onSuccess, allowRegistrations }: Props)
           required
           fullWidth
           size="small"
-          disabled={!allowRegistrations}
         />
-        <Button type="submit" variant="contained" disabled={loading || !allowRegistrations} fullWidth size="large">
+        <Button type="submit" variant="contained" disabled={loading} fullWidth size="large">
           {loading ? t("auth.register.submitting") : t("auth.register.submit")}
         </Button>
       </Stack>
diff --git a/frontend/src/pages/AuthPage/index.tsx b/frontend/src/pages/AuthPage/index.tsx
index e15699d..77c54ec 100644
--- a/frontend/src/pages/AuthPage/index.tsx
+++ b/frontend/src/pages/AuthPage/index.tsx
@@ -10,17 +10,23 @@ import Alert from "@mui/material/Alert";
 import type { AuthUser } from "../../api/auth";
 import Wordmark from "../../components/Wordmark";
 import SignInPanel from "./SignInPanel";
-import RegisterPanel from "./RegisterPanel";
+import RegisterPanel, { type Invite } from "./RegisterPanel";
 
 type Props = {
   onSuccess: (token: string, expires_in: number, user: AuthUser) => void;
   apiUp: boolean | null;
   allowRegistrations: boolean;
+  /** From an invitation link -- opens on the register form for that address, even while
+   *  registrations are closed. */
+  invite?: Invite | null;
 };
 
-export default function AuthPage({ onSuccess, apiUp, allowRegistrations }: Props) {
+export default function AuthPage({ onSuccess, apiUp, allowRegistrations, invite = null }: Props) {
   const { t } = useTranslation("app");
-  const [tab, setTab] = React.useState<"signIn" | "register">("signIn");
+  const [tab, setTab] = React.useState<"signIn" | "register">(invite ? "register" : "signIn");
+  // Closed registrations drop the Register tab altogether -- there's nothing to fill in -- unless
+  // this visit came from an invitation.
+  const canRegister = allowRegistrations || invite !== null;
 
   return (
     <Paper
@@ -49,20 +55,22 @@ export default function AuthPage({ onSuccess, apiUp, allowRegistrations }: Props
         </Alert>
       )}
 
-      <Tabs
-        value={tab}
-        onChange={(_e, value) => setTab(value)}
-        variant="fullWidth"
-        sx={{ mb: 3, borderBottom: "1px solid", borderColor: "divider" }}
-      >
-        <Tab value="signIn" label={t("auth.signInTab")} />
-        <Tab value="register" label={t("auth.registerTab")} />
-      </Tabs>
+      {canRegister && (
+        <Tabs
+          value={tab}
+          onChange={(_e, value) => setTab(value)}
+          variant="fullWidth"
+          sx={{ mb: 3, borderBottom: "1px solid", borderColor: "divider" }}
+        >
+          <Tab value="signIn" label={t("auth.signInTab")} />
+          <Tab value="register" label={t("auth.registerTab")} />
+        </Tabs>
+      )}
 
-      {tab === "signIn" ? (
-        <SignInPanel onSuccess={onSuccess} />
+      {tab === "register" && canRegister ? (
+        <RegisterPanel onSuccess={onSuccess} invite={invite} />
       ) : (
-        <RegisterPanel onSuccess={onSuccess} allowRegistrations={allowRegistrations} />
+        <SignInPanel onSuccess={onSuccess} />
       )}
     </Paper>
   );
diff --git a/frontend/src/pages/LogsPage/index.tsx b/frontend/src/pages/LogsPage/index.tsx
index 56f96f2..ec57711 100644
--- a/frontend/src/pages/LogsPage/index.tsx
+++ b/frontend/src/pages/LogsPage/index.tsx
@@ -30,6 +30,7 @@ type ActionColor = "success" | "error" | "info" | "warning" | "default";
 const ACTION_COLORS: Record<LogAction, ActionColor> = {
   user_logged_in: "success",
   user_logged_out: "default",
+  user_invited: "info",
   model_uploaded: "info",
   model_imported: "info",
   import_completed: "info",
@@ -102,6 +103,8 @@ export default function LogsPage({ onUnauthorized }: Props) {
       case "collection_item_added":
       case "collection_item_removed":
         return typeof d.name === "string" ? d.name : "";
+      case "user_invited":
+        return typeof d.email === "string" ? d.email : "";
       case "model_edited":
         return typeof d.field === "string" ? t("adminSettings.logs.editedField", { field: d.field }) : "";
       case "import_completed": {
diff --git a/frontend/src/pages/UsersPage/InviteUsersDialog.tsx b/frontend/src/pages/UsersPage/InviteUsersDialog.tsx
new file mode 100644
index 0000000..358d35e
--- /dev/null
+++ b/frontend/src/pages/UsersPage/InviteUsersDialog.tsx
@@ -0,0 +1,82 @@
+import React from "react";
+import { useTranslation } from "react-i18next";
+import Alert from "@mui/material/Alert";
+import Button from "@mui/material/Button";
+import Dialog from "@mui/material/Dialog";
+import DialogActions from "@mui/material/DialogActions";
+import DialogContent from "@mui/material/DialogContent";
+import DialogContentText from "@mui/material/DialogContentText";
+import DialogTitle from "@mui/material/DialogTitle";
+import Stack from "@mui/material/Stack";
+import TextField from "@mui/material/TextField";
+import { adminApi } from "../../api/admin";
+
+type Props = {
+  open: boolean;
+  onClose: () => void;
+};
+
+/** Emails an invitation to register while registrations are closed (see RegistrationsPanel).
+ *  Stays open after a send, with the field cleared, so several people can be invited in a row. */
+export default function InviteUsersDialog({ open, onClose }: Props) {
+  const { t } = useTranslation("app");
+  const [email, setEmail] = React.useState("");
+  const [sending, setSending] = React.useState(false);
+  const [sent, setSent] = React.useState<{ email: string; days: number } | null>(null);
+  const [error, setError] = React.useState<string | null>(null);
+
+  React.useEffect(() => {
+    if (!open) return;
+    setEmail("");
+    setSent(null);
+    setError(null);
+  }, [open]);
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+    if (!email.trim() || sending) return;
+    setSending(true);
+    setError(null);
+    setSent(null);
+    try {
+      const res = await adminApi.inviteUser(email.trim());
+      setSent({ email: res.email, days: res.expires_in_days });
+      setEmail("");
+    } catch (err) {
+      setError(err instanceof Error ? err.message : t("adminSettings.registrations.invite.failed"));
+    } finally {
+      setSending(false);
+    }
+  };
+
+  return (
+    <Dialog open={open} onClose={sending ? undefined : onClose} fullWidth maxWidth="xs">
+      <form onSubmit={handleSubmit}>
+        <DialogTitle>{t("adminSettings.registrations.invite.title")}</DialogTitle>
+        <DialogContent>
+          <Stack spacing={2}>
+            <DialogContentText>{t("adminSettings.registrations.invite.description")}</DialogContentText>
+            {sent && <Alert severity="success">{t("adminSettings.registrations.invite.sent", { email: sent.email, count: sent.days })}</Alert>}
+            {error && <Alert severity="error">{error}</Alert>}
+            <TextField
+              type="email"
+              label={t("adminSettings.registrations.invite.emailLabel")}
+              value={email}
+              onChange={e => setEmail(e.target.value)}
+              required
+              fullWidth
+              size="small"
+              disabled={sending}
+            />
+          </Stack>
+        </DialogContent>
+        <DialogActions>
+          <Button onClick={onClose} disabled={sending}>{t("adminSettings.registrations.invite.close")}</Button>
+          <Button type="submit" variant="contained" disabled={sending || !email.trim()}>
+            {sending ? t("adminSettings.registrations.invite.sending") : t("adminSettings.registrations.invite.send")}
+          </Button>
+        </DialogActions>
+      </form>
+    </Dialog>
+  );
+}
diff --git a/frontend/src/pages/UsersPage/RegistrationsPanel.tsx b/frontend/src/pages/UsersPage/RegistrationsPanel.tsx
new file mode 100644
index 0000000..65093f6
--- /dev/null
+++ b/frontend/src/pages/UsersPage/RegistrationsPanel.tsx
@@ -0,0 +1,113 @@
+import React from "react";
+import { useTranslation } from "react-i18next";
+import Alert from "@mui/material/Alert";
+import Button from "@mui/material/Button";
+import CircularProgress from "@mui/material/CircularProgress";
+import FormControlLabel from "@mui/material/FormControlLabel";
+import Paper from "@mui/material/Paper";
+import Stack from "@mui/material/Stack";
+import Switch from "@mui/material/Switch";
+import Typography from "@mui/material/Typography";
+import PersonAddIcon from "@mui/icons-material/PersonAddAlt1";
+import { UnauthorizedError } from "../../api/client";
+import { settingsApi } from "../../api/settings";
+import InviteUsersDialog from "./InviteUsersDialog";
+
+type Props = {
+  onUnauthorized?: () => void;
+};
+
+/** Instance-wide: whether anyone may create an account, shown above the Users table. Turned off,
+ *  the sign-in page drops its Register tab and the backend refuses registrations -- except the very
+ *  first account, and people invited from here. Inviting needs SMTP (Connections > SMTP), since
+ *  the link travels by email, so the button only appears once registrations are closed and SMTP is
+ *  set up. */
+export default function RegistrationsPanel({ onUnauthorized }: Props) {
+  const { t } = useTranslation("app");
+  const [allow, setAllow] = React.useState(true);
+  const [smtpConfigured, setSmtpConfigured] = React.useState(false);
+  const [loading, setLoading] = React.useState(true);
+  const [saving, setSaving] = React.useState(false);
+  const [error, setError] = React.useState<string | null>(null);
+  const [inviteOpen, setInviteOpen] = React.useState(false);
+
+  React.useEffect(() => {
+    let active = true;
+    void (async () => {
+      try {
+        const [registrations, smtp] = await Promise.all([settingsApi.getRegistrations(), settingsApi.getSmtp()]);
+        if (!active) return;
+        setAllow(registrations.allow_registrations);
+        setSmtpConfigured(smtp.configured);
+      } catch (err) {
+        if (err instanceof UnauthorizedError) onUnauthorized?.();
+      } finally {
+        if (active) setLoading(false);
+      }
+    })();
+    return () => { active = false; };
+  }, [onUnauthorized]);
+
+  const handleChange = async (next: boolean) => {
+    const previous = allow;
+    setAllow(next);
+    setSaving(true);
+    setError(null);
+    try {
+      const res = await settingsApi.updateRegistrations(next);
+      setAllow(res.allow_registrations);
+    } catch (err) {
+      if (err instanceof UnauthorizedError) {
+        onUnauthorized?.();
+        return;
+      }
+      setAllow(previous);
+      setError(err instanceof Error ? err.message : t("adminSettings.registrations.failed"));
+    } finally {
+      setSaving(false);
+    }
+  };
+
+  const canInvite = !allow && smtpConfigured;
+
+  return (
+    <Paper variant="outlined" sx={{ p: 2.5 }}>
+      <Stack spacing={1.5}>
+        <Stack direction={{ xs: "column", sm: "row" }} spacing={2} alignItems={{ sm: "center" }} justifyContent="space-between">
+          <Stack spacing={0.5}>
+            <Stack direction="row" spacing={1.5} alignItems="center">
+              <FormControlLabel
+                control={
+                  <Switch
+                    checked={allow}
+                    disabled={loading || saving}
+                    onChange={e => void handleChange(e.target.checked)}
+                  />
+                }
+                label={t("adminSettings.registrations.allowLabel")}
+              />
+              {saving && <CircularProgress size={16} />}
+            </Stack>
+            <Typography variant="body2" color="text.secondary">
+              {allow ? t("adminSettings.registrations.openHelp") : t("adminSettings.registrations.closedHelp")}
+            </Typography>
+          </Stack>
+
+          {canInvite && (
+            <Stack spacing={0.5} alignItems={{ xs: "flex-start", sm: "flex-end" }} sx={{ flexShrink: 0 }}>
+              <Button variant="contained" startIcon={<PersonAddIcon />} onClick={() => setInviteOpen(true)}>
+                {t("adminSettings.registrations.inviteButton")}
+              </Button>
+              <Typography variant="caption" color="text.secondary">
+                {t("adminSettings.registrations.inviteHelp")}
+              </Typography>
+            </Stack>
+          )}
+        </Stack>
+
+        {error && <Alert severity="error" onClose={() => setError(null)}>{error}</Alert>}
+      </Stack>
+      <InviteUsersDialog open={inviteOpen} onClose={() => setInviteOpen(false)} />
+    </Paper>
+  );
+}
diff --git a/frontend/src/pages/UsersPage/index.tsx b/frontend/src/pages/UsersPage/index.tsx
index 79d89c6..dc04f59 100644
--- a/frontend/src/pages/UsersPage/index.tsx
+++ b/frontend/src/pages/UsersPage/index.tsx
@@ -13,12 +13,14 @@ import TableHead from "@mui/material/TableHead";
 import TableRow from "@mui/material/TableRow";
 import { UnauthorizedError } from "../../api/client";
 import { adminApi, type AdminUser } from "../../api/admin";
+import RegistrationsPanel from "./RegistrationsPanel";
 
 type Props = {
   onUnauthorized?: () => void;
 };
 
-/** Admin-only, read-only roster of every account on the instance. Models/Collections are
+/** Admin-only roster of every account on the instance, under the instance-wide registration
+ *  switch and invitations (RegistrationsPanel). The roster itself is read-only. Models/Collections are
  *  per-user counts; MakerWorld is a connected/not-connected flag (each user's own MakerWorld
  *  session cookie, saved server-side from Settings -> Imports -- see
  *  services/makerworldCookieService.ts). The "delete all models for a user" action lives on the
@@ -53,6 +55,8 @@ export default function UsersPage({ onUnauthorized }: Props) {
 
   return (
     <Stack spacing={3}>
+      <RegistrationsPanel onUnauthorized={onUnauthorized} />
+
       {error && <Alert severity="error" onClose={() => setError(null)}>{error}</Alert>}
 
       {loading ? (