Cool idea on how to manage spam and quality in small, focused communities from Lobste.rs, a new-to-me link aggregator that’s kind of like Hacker News: They use an invite “tree” to publicly show who invited who (whom?). That way, once a year, you can always see how someone got into the system. From their “about” section:

Not for exclusivity, but rather, invitations will be used as a spam-control mechanism. New users must be invited by a current member and invitations will be unlimited (unless scaling problems temporarily prevent new accounts). If spammers are invited to the site and banned, the user that invited them may also be banned, going up the chain of invitations as needed.