"Existing Gmail users will be allocated invitations to invite friends and other people to have a spin at it. However, the allocation system may not be as random as it seems - it's quite likely that there's a direct tie between the number of invitations you're given and/or the frequency of its occurrence with and the amount of space you've used up in your account. See, I sent Bernard an invitation and two days later, he has got 3 of his own to send out. And he reckoned that it's probably because of the amount of mails he'd received - he'd signed up for all the 'misc' subscriptions and definitely had no problem filling up his inbox. I realised that the same happened for me. By the time I'd used up about 6MB, I had 3 to send out, and at around 11MB, I had 3 more.
So, in order to get more invites, invite spam
"
http://n1kki.frens.net/archives/2004/06/17/how_to_get_gmail_invites.html
there you go, fill up space