Any kind of invite system provides an easy and precise statistics on account relations. I.E. if you see a bunch of accounts being banned recently and a lot of them used an invite from a particular account, then it's safe to assume that account invites bad actors or is compromised, without any formal audit system. It can be effectively automated and easily run by a single admin to boot. As a side effect it also controls the influx of members, as it prevents an unintended slashdot effect. For example, GMail, a gigantic "free" ad-ridden bloatware as known today, started as an .