iirc there was a proposal by nostr:npub1v0lxxxxutpvrelsksy8cdhgfux9l6a42hsj2qzquu2zk7vc9qnkszrqj49 to add nchat references, they probably can be used to reference groupchats
Discussion
I think nip29 groups should have standard unique ids just like our pubkeys (nprofile), something like `ngroup` which also includes its relay url.