diff --git a/.env.example b/.env.example
index 0c784bd..94541e9 100644
--- a/.env.example
+++ b/.env.example
@@ -1,3 +1,4 @@
 PUBLIC_RELAY_URL=ws://localhost:3334
 PUBLIC_GROUP_ID=mygrouprandomid
 PUBLIC_MODE=simple   # simple | full
+PUBLIC_JOINCODE=no   # yes | no — show invite-code field on join failure
diff --git a/src/lib/auth.svelte.ts b/src/lib/auth.svelte.ts
index e792844..ebc8df6 100644
--- a/src/lib/auth.svelte.ts
+++ b/src/lib/auth.svelte.ts
@@ -3,6 +3,7 @@ import type { WindowNostr } from "@nostr/tools/nip07";
 import type { EventTemplate, VerifiedEvent } from "@nostr/tools/core";
 import * as nip19 from "@nostr/tools/nip19";
 import { finalizeEvent, getPublicKey } from "@nostr/tools/pure";
+import { resetJoinState, initJoinForUser } from "$lib/join.svelte";
 
 declare global {
   interface Window {
@@ -58,6 +59,7 @@ function makeNsecSigner(secretKey: Uint8Array): Signer {
 async function setUser(pubkey: string) {
   const { loadNostrUser } = await import("@nostr/gadgets/metadata");
   user = await loadNostrUser(pubkey);
+  initJoinForUser(pubkey);
 }
 
 export async function loginWithExtension() {
@@ -108,6 +110,7 @@ export function logout() {
   localStorage.removeItem(PUBKEY_KEY);
   localStorage.removeItem(METHOD_KEY);
   localStorage.removeItem(NSEC_KEY);
+  resetJoinState();
 }
 
 export async function restoreSession() {
diff --git a/src/lib/components/JoinModal.svelte b/src/lib/components/JoinModal.svelte
new file mode 100644
index 0000000..d8c0fb3
--- /dev/null
+++ b/src/lib/components/JoinModal.svelte
@@ -0,0 +1,115 @@
+<script lang="ts">
+  import { joinState, closeJoinModal, retryJoin } from "$lib/join.svelte";
+  import { tick } from "svelte";
+
+  let code = $state("");
+  let codeInput = $state<HTMLInputElement | null>(null);
+
+  $effect(() => {
+    if (joinState.modalOpen && joinState.codeRequired) {
+      tick().then(() => codeInput?.focus());
+    }
+    if (!joinState.modalOpen) code = "";
+  });
+
+  async function onRetry() {
+    await retryJoin(code.trim() || undefined);
+  }
+
+  function onClose() {
+    closeJoinModal();
+  }
+
+  function onKeydown(e: KeyboardEvent) {
+    if (!joinState.modalOpen) return;
+    if (e.key === "Escape") onClose();
+  }
+</script>
+
+<svelte:window onkeydown={onKeydown} />
+
+{#if joinState.modalOpen}
+  <div class="fixed inset-0 z-50 flex items-center justify-center p-4">
+    <button
+      type="button"
+      aria-label="Close"
+      class="absolute inset-0 bg-black/40"
+      onclick={onClose}
+    ></button>
+    <div
+      class="relative w-full max-w-md rounded-lg bg-white p-6 shadow-xl"
+      role="dialog"
+      aria-modal="true"
+      aria-labelledby="join-title"
+    >
+      <button
+        type="button"
+        onclick={onClose}
+        aria-label="Close"
+        disabled={joinState.busy}
+        class="absolute right-3 top-3 text-2xl leading-none text-gray-400 hover:text-gray-700 disabled:opacity-50"
+      >
+        ×
+      </button>
+
+      <h2 id="join-title" class="mb-2 text-lg font-semibold text-gray-900">
+        Join this group
+      </h2>
+      <p class="mb-4 text-sm text-gray-600">
+        {#if joinState.codeRequired}
+          This community requires an invite code. Enter your code below to
+          request access. If you don't have one, contact the admin.
+        {:else}
+          We could not add you to the group automatically. The relay may be
+          processing your request, or the admin needs to approve it manually.
+        {/if}
+      </p>
+
+      {#if joinState.modalError}
+        <div
+          class="mb-4 rounded border border-red-200 bg-red-50 px-3 py-2 text-sm text-red-700"
+          role="alert"
+        >
+          {joinState.modalError}
+        </div>
+      {/if}
+
+      {#if joinState.codeRequired}
+        <label for="join-code-input" class="sr-only">Invite code</label>
+        <input
+          id="join-code-input"
+          bind:this={codeInput}
+          type="text"
+          placeholder="Invite code"
+          bind:value={code}
+          disabled={joinState.busy}
+          autocomplete="off"
+          autocapitalize="off"
+          spellcheck="false"
+          onkeydown={(e) => e.key === "Enter" && onRetry()}
+          class="mb-3 w-full rounded border border-gray-200 px-3 py-2 text-sm focus:outline-none focus:ring-1 focus:ring-brand disabled:opacity-50"
+        />
+      {/if}
+
+      <div class="flex justify-end gap-2">
+        <button
+          type="button"
+          onclick={onClose}
+          disabled={joinState.busy}
+          class="rounded px-3 py-2 text-sm font-medium text-gray-600 hover:bg-gray-100 disabled:opacity-50"
+        >
+          Close
+        </button>
+        <button
+          type="button"
+          onclick={onRetry}
+          disabled={joinState.busy ||
+            (joinState.codeRequired && !code.trim())}
+          class="rounded bg-brand px-3 py-2 text-sm font-medium text-white hover:bg-brand-hover disabled:cursor-not-allowed disabled:opacity-50"
+        >
+          {joinState.busy ? "Trying…" : "Retry"}
+        </button>
+      </div>
+    </div>
+  </div>
+{/if}
diff --git a/src/lib/config.ts b/src/lib/config.ts
index 0325f3c..d3a6825 100644
--- a/src/lib/config.ts
+++ b/src/lib/config.ts
@@ -2,9 +2,11 @@ import {
   PUBLIC_RELAY_URL,
   PUBLIC_GROUP_ID,
   PUBLIC_MODE,
+  PUBLIC_JOINCODE,
 } from "$env/static/public";
 
 export const RELAY_URL = PUBLIC_RELAY_URL;
 export const GROUP_ID = PUBLIC_GROUP_ID;
 export const MODE: "simple" | "full" =
   PUBLIC_MODE === "full" ? "full" : "simple";
+export const JOINCODE_REQUIRED = PUBLIC_JOINCODE === "yes";
diff --git a/src/lib/join.svelte.ts b/src/lib/join.svelte.ts
new file mode 100644
index 0000000..be50ceb
--- /dev/null
+++ b/src/lib/join.svelte.ts
@@ -0,0 +1,172 @@
+import { Relay, SimplePool } from "@nostr/tools";
+import { auth } from "$lib/auth.svelte";
+import { GROUP_ID, RELAY_URL, JOINCODE_REQUIRED } from "$lib/config";
+
+let joined = $state(false);
+let modalOpen = $state(false);
+let modalError = $state<string | null>(null);
+let busy = $state(false);
+let pendingAction: (() => Promise<void>) | null = null;
+
+export const joinState = {
+  get joined() {
+    return joined;
+  },
+  get modalOpen() {
+    return modalOpen;
+  },
+  get modalError() {
+    return modalError;
+  },
+  get busy() {
+    return busy;
+  },
+  get codeRequired() {
+    return JOINCODE_REQUIRED;
+  },
+};
+
+export function resetJoinState() {
+  joined = false;
+  modalOpen = false;
+  modalError = null;
+  busy = false;
+  pendingAction = null;
+}
+
+// Checks group membership: kind:9000 (per-user put-user event, lightweight)
+// first, falling back to kind:39002 (full members list, heavier) only if 9000
+// did not match. Either signal marks the user as joined and skips the 9021.
+function queryHasMatch(
+  relay: Relay,
+  filter: Parameters<Relay["subscribe"]>[0][number],
+  timeoutMs = 3000,
+): Promise<boolean> {
+  return new Promise((resolve) => {
+    let settled = false;
+    const finish = (val: boolean) => {
+      if (settled) return;
+      settled = true;
+      try {
+        sub.close();
+      } catch {}
+      resolve(val);
+    };
+    const sub = relay.subscribe([filter], {
+      onevent() {
+        finish(true);
+      },
+      oneose() {
+        finish(false);
+      },
+      onclose() {
+        finish(false);
+      },
+    });
+    setTimeout(() => finish(false), timeoutMs);
+  });
+}
+
+export async function initJoinForUser(pubkey: string) {
+  let relay: Relay;
+  try {
+    relay = await Relay.connect(RELAY_URL);
+  } catch {
+    return;
+  }
+  try {
+    const has9000 = await queryHasMatch(relay, {
+      kinds: [9000],
+      "#h": [GROUP_ID],
+      "#p": [pubkey],
+      limit: 1,
+    });
+    if (has9000) {
+      joined = true;
+      return;
+    }
+    const has39002 = await queryHasMatch(relay, {
+      kinds: [39002],
+      "#d": [GROUP_ID],
+      "#p": [pubkey],
+      limit: 1,
+    });
+    if (has39002) joined = true;
+  } finally {
+    try {
+      relay.close();
+    } catch {}
+  }
+}
+
+export function closeJoinModal() {
+  if (busy) return;
+  modalOpen = false;
+  modalError = null;
+  pendingAction = null;
+}
+
+async function publishJoinRequest(code?: string) {
+  if (!auth.signer) throw new Error("Not logged in");
+  const tags: string[][] = [["h", GROUP_ID]];
+  if (code) tags.push(["code", code]);
+  const event = await auth.signer.signEvent({
+    kind: 9021,
+    created_at: Math.floor(Date.now() / 1000),
+    tags,
+    content: "",
+  });
+  const pool = new SimplePool();
+  try {
+    const timeout = new Promise<never>((_, reject) =>
+      setTimeout(() => reject(new Error("Relay did not respond in time")), 8000),
+    );
+    await Promise.race([
+      Promise.all(pool.publish([RELAY_URL], event)),
+      timeout,
+    ]);
+  } finally {
+    pool.destroy();
+  }
+}
+
+// Wraps a group action. If not yet joined, sends kind:9021 first, then the
+// action. On failure, opens the join modal so the user can retry (with code if
+// configured). Returns true on success, false if the modal was opened.
+export async function withJoin(action: () => Promise<void>): Promise<boolean> {
+  if (joined) {
+    await action();
+    return true;
+  }
+  busy = true;
+  try {
+    await publishJoinRequest();
+    await action();
+    joined = true;
+    return true;
+  } catch (e) {
+    pendingAction = action;
+    modalError = e instanceof Error ? e.message : "Could not join the group";
+    modalOpen = true;
+    return false;
+  } finally {
+    busy = false;
+  }
+}
+
+export async function retryJoin(code?: string) {
+  if (!pendingAction || busy) return;
+  busy = true;
+  modalError = null;
+  try {
+    await publishJoinRequest(code);
+    await pendingAction();
+    joined = true;
+    modalOpen = false;
+    pendingAction = null;
+  } catch (e) {
+    modalError = e instanceof Error ? e.message : "Could not join the group";
+  } finally {
+    busy = false;
+  }
+}
diff --git a/src/routes/+layout.svelte b/src/routes/+layout.svelte
index 09c0421..043c043 100644
--- a/src/routes/+layout.svelte
+++ b/src/routes/+layout.svelte
@@ -5,6 +5,7 @@
   import LeftSidebar from "$lib/components/LeftSidebar.svelte";
   import ChatSidebar from "$lib/components/ChatSidebar.svelte";
   import LoginModal from "$lib/components/LoginModal.svelte";
+  import JoinModal from "$lib/components/JoinModal.svelte";
   import { page } from "$app/state";
   import { onMount } from "svelte";
   import { restoreSession } from "$lib/auth.svelte";
@@ -48,3 +49,4 @@
 </div>
 
 <LoginModal />
+<JoinModal />
diff --git a/src/routes/thread/[id]/+page.svelte b/src/routes/thread/[id]/+page.svelte
index 12eca35..ef3b404 100644
--- a/src/routes/thread/[id]/+page.svelte
+++ b/src/routes/thread/[id]/+page.svelte
@@ -8,6 +8,7 @@
     type PostData,
   } from "$lib/thread.svelte";
   import { auth, openLogin } from "$lib/auth.svelte";
+  import { withJoin } from "$lib/join.svelte";
   import Reactions from "$lib/components/Reactions.svelte";
   import ThreadScrubber from "$lib/components/ThreadScrubber.svelte";
   import type { NostrUser } from "@nostr/gadgets/metadata";
@@ -43,11 +44,15 @@
 
   async function submitReply() {
     if (!auth.user || !replyContent.trim()) return;
+    const content = replyContent.trim();
+    const pubkey = auth.user.pubkey;
     replying = true;
     replyError = null;
     try {
-      await sendReply(replyContent.trim(), auth.user!.pubkey);
-      replyContent = "";
+      await withJoin(async () => {
+        await sendReply(content, pubkey);
+        replyContent = "";
+      });
     } catch (e) {
       replyError = e instanceof Error ? e.message : "Failed to post reply";
     } finally {