Yes, we had an invite code system when we started to limit growth to what we could support! But it's less effective for curbing spam than you'd think: people generally share invites widely enough that it's easy for spammers to get them, and all it does is put a barrier in front of people who are looking to make legitimate accounts. It's also enough of a pain in the ass, administratively, that we were really happy to ditch it when we could. If it were more effective in curbing spam registration, absolutely we would turn it back on, but the combination of "not great at spam fighting" + "deeply annoying for legitimate use" + "just as much of an administrative burden as the spam fighting" puts it pretty far down on the list. (At most, we'd require invites for creating accounts from one of the six countries that 90% of our spam comes from, but that's 90% of the admin annoyance as requiring them for everyone so we haven't yet.)