diff --git a/app/main.py b/app/main.py
index db176e8..45b95bc 100644
--- a/app/main.py
+++ b/app/main.py
@@ -451,6 +451,12 @@ def get_db() -> sqlite3.Connection:
     # used to throttle re-invites of members who never activated.
     if "welcome_sent_at" not in cols:
         conn.execute("ALTER TABLE members ADD COLUMN welcome_sent_at TEXT")
+    # Migration: matrix_mxid — an OPTIONAL extra Matrix account a member wants
+    # invited to the co-op Space (e.g. their existing federated account on
+    # another homeserver). Empty = only the local @username:inference.coop
+    # account is invited. At most one extra MXID per member.
+    if "matrix_mxid" not in cols:
+        conn.execute("ALTER TABLE members ADD COLUMN matrix_mxid TEXT")
     # Migration: credit_balance — purchased, NON-EXPIRING credits (USD).
     # Distinct from `balance` (the monthly allowance, which resets): credits
     # carry over indefinitely and are spent only after the allowance is
@@ -787,6 +793,27 @@ async def get_active_member_emails() -> list[str]:
     return [r[0] for r in rows]
 
 
+def get_member_matrix_mxid(email: str) -> str:
+    """Return the member's optional extra Matrix MXID, or "" if unset."""
+    conn = get_db()
+    row = conn.execute(
+        "SELECT matrix_mxid FROM members WHERE email = ?", (email,)
+    ).fetchone()
+    conn.close()
+    return (row[0] or "").strip() if row else ""
+
+
+def set_member_matrix_mxid(email: str, mxid: str) -> None:
+    """Set (or clear) the member's optional extra Matrix MXID."""
+    conn = get_db()
+    conn.execute(
+        "UPDATE members SET matrix_mxid = ? WHERE email = ?",
+        (mxid.strip() if mxid else "", email),
+    )
+    conn.commit()
+    conn.close()
+
+
 async def loomio_fix_usernames(client: httpx.AsyncClient) -> dict:
     """Rename Loomio users with auto-generated handles to their Cloudron username.
 
@@ -922,6 +949,15 @@ async def matrix_member_mxids(client: httpx.AsyncClient):
             break
         page += 1
     for email in await get_active_member_emails():
+        # A member's own extra MXID (set on the dashboard) takes precedence —
+        # that's the account they want in the Space. Otherwise use their local
+        # @username:inference.coop account.
+        own = get_member_matrix_mxid(email)
+        if own:
+            if own not in seen:
+                seen.add(own)
+                yield email, own
+            continue
         username = users.get(email.lower())
         if username:
             mxid = f"@{username}:inference.coop"
@@ -2332,6 +2368,59 @@ async def broker_revoke(name: str, request: Request):
     return {"status": "revoked", "name": name}
 
 
+# Matrix MXID (self-serve federated identity). A member can attach ONE extra
+# Matrix account (e.g. their existing @user:matrix.org) to be invited to the
+# co-op Space. Empty string clears it. Validation is loose on purpose — Synapse
+# will simply reject a malformed MXID on invite, and we log that.
+_MXID_RE = re.compile(r"^@[^\s:]+:[^\s]+\.[^\s]+$")
+
+
+@app.get("/broker/matrix-mxid")
+@limiter.limit("30/minute")
+async def broker_get_matrix_mxid(request: Request):
+    email = _verify_broker_secret(request)
+    return {"matrix_mxid": get_member_matrix_mxid(email), "local_mxid": local_mxid_for_email(email)}
+
+
+async def local_mxid_for_email(email: str) -> str:
+    """Resolve a member's local Matrix ID (@username:inference.coop), or '' if unknown."""
+    import urllib.request
+    try:
+        req = urllib.request.Request(
+            f"{CLOUDRON_API}/api/v1/users?per_page=100",
+            headers={"Authorization": f"Bearer {CLOUDRON_TOKEN}"},
+        )
+        with urllib.request.urlopen(req, timeout=15) as r:
+            users = json.loads(r.read().decode()).get("users", [])
+        for u in users:
+            if (u.get("email") or "").lower() == email and u.get("username"):
+                return f"@{u['username']}:inference.coop"
+    except Exception as e:
+        logger.warning("local_mxid_for_email lookup failed: %s", e)
+    return ""
+
+
+@app.post("/broker/matrix-mxid")
+@limiter.limit("30/minute")
+async def broker_set_matrix_mxid(request: Request):
+    email = _verify_broker_secret(request)
+    body = await request.json()
+    mxid = (body.get("matrix_mxid") or "").strip()
+    if mxid and not _MXID_RE.match(mxid):
+        return JSONResponse(
+            {"error": "Enter a Matrix ID like @username:server.org"}, status_code=400
+        )
+    set_member_matrix_mxid(email, mxid)
+    # Invite the new account into the Space immediately (best-effort; the daily
+    # sync will also pick it up). Clear = no invite (just removes the mapping).
+    if mxid:
+        try:
+            await sync_matrix_space()
+        except Exception as e:
+            logger.warning("matrix-mxid: immediate invite failed: %s", e)
+    return {"status": "saved", "matrix_mxid": mxid}
+
+
 async def broker_get_usage(email: str, days: int = 0) -> dict:
     """Return the member's spend vs. balance, plus per-model and token detail.