It would be possible to make it purely user-side subjective by each user approving memberships in a group message broadcast). A moderated (read, centralised) group message protocol has an administrator.

I think this would be more nostry :D

If the group is just a random 256 bit value (not a key) then the only problem might be in an overwhelming number of messages to comb through to approve to watch. A protocol to copy another user's watch would solve that in the bootstrap case I guess.