diff --git a/29.md b/29.md
index 3593c1f804..15191e6eea 100644
--- a/29.md
+++ b/29.md
@@ -22,6 +22,14 @@ Relays are supposed to generate the events that describe group metadata and grou
 
 A group may be identified by a `naddr1...` code to its `kind:39000` metadata event, with the public key set to the relay `self` public key, the `d` identifier set to the group id, the kind set to `39000` and a relay hint to the relay that is hosting the group.
 
+An invite code may be appended to the group identifier:
+
+```
+naddr1...?invite=<code>
+```
+
+Clients should use this code in the `code` tag of the `kind:9021` join request. Since the bech32 charset doesn't include `?`, everything before the `?` is still a valid identifier on its own, so clients that don't understand the suffix can just ignore it.
+
 ## The `h` tag
 
 Events sent by users to groups (chat messages, text notes, moderation events etc) MUST have an `h` tag with the value set to the group _id_.