I've heard a little about gmail, but what do you mean by "invites"?
In my opinion it is Google's very innovative "rumour" marketing idea. They are trying to convince users that the access to their service is restricted to the invited people only. Funny, but it seems to work .
This site is an unofficial clearinghouse for Gmail invites. I've sent 100+ invites to them already. As of right now, they have 108,253 invitations to dispense. This is probably better than posting your current email address on a public bulletin board...