It was back when we were still in beta (2009-2011 or so)! We would generate batches of invite codes semi-regularly based on certain criteria -- like, sometimes we'd send codes to people who'd posted recently, sometimes to people who'd logged in recently, sometimes to people who'd used all their invites, sometimes to people who'd invited people who were active on the site, yadda. We had a bunch of different methods we could use to hand them out and we tried to make sure everybody who was active on the site had at least one or two on hand at all times, plus we could make 'bulk' codes that could be used to create a larger number of accounts (like, if you were starting a new RP game and wanted to make sure all your players could make accounts for it, we could give you a single code that worked to make 50 accounts and not just one, etc). It was a good method for making sure that we didn't overwhelm the site completely when we launched, but it was a lot of administrative overhead and we were glad to finally ditch them!