You write a simple bot that sends out gmail invites to a mail server it has access to (the only thing this email server is used for is to collect invites). It keeps track how many invites it has to give out and dispenses invites to itself sets up the email accounts for users then harvests all invites in that account. For every user you set up you get 50 more invites. If invites expire you factor that into the bots behavior.
PS: Dear Google staff I would never consider doing this please don't lock down my gmail account. I do love it so.