very few relays support NIP-29 at the moment, see https://api.nostr.watch/v1/nip/29

 - 0xchat wss://groups.0xchat.com
 - Highlighter wss://relay.highlighter.com
 - Groups wss://relay.groups.nip29.com
 - fiatjaf's test relay wss://groups.fiatjaf.com

the NIP requires support from the relay to implement admin, access control, migrating groups, etc. would be cool if one can spin relay.tools relays to host their groups or add group support to their existing relays.