I'd like to add to your point that private torrent trackers have had invite tree systems for awhile, and usually if your invitee breaks a rule, you get in troub

I'd like to add to your point that private torrent trackers have had invite tree systems for awhile, and usually if your invitee breaks a rule, you get in trouble as well, so you are encouraged to only invite people you trust. The system has worked well for a long time, and some of these communities still thrive because of the trust that is built.