It's a lot simpler than in a twitter context. Since the relay + identifier is the full id of the group, forking happens explicitly by copying data and using a new qualified id. The outbox model in the narrow sense is irrelevant, and in a broader sense it's barely visible (unless nostr:nprofile1qythwumn8ghj7un9d3shjtnswf5k6ctv9ehx2ap0qy2hwumn8ghj7un9d3shjtnyv9kh2uewd9hj7qg6waehxw309ac8junpd45kgtnxd9shg6npvchxxmmd9uqs6amnwvaz7tmxxaazu6t09uq32amnwvaz7tmpv4nkjueww468smewdahx2tcqyrafsj7hmweg9ur7zmn6apajdg48hxuskujx53rhrux0ttjcqx84y4vxupk starts building multi-relay groups, then all bets are off).
