[SV_BubbleTime]

And when you need longer reach than that?

A “I vouch for this person” system?

[Pay08]

That has largely worked for private trackers.

[shit_game]

lobste.rs has a pretty decent system with a global invite tree, where users can provide access for other people. it comes with the benefit of creating an association graph of accounts that allows for swift moderation, and lets the userbase grow within a community of people likely to appreciate the culture.