Google is throttling the invites. I can understand why they would want to control the deluge of people wanting to join. Also, the whole needing an invite thing makes it feel exclusive and people want what they can't have. ;-)
Oddly, I'm in with a bunch of my peeps from Europe and Asia (28 people). I am only one experience, but it looks like they are favoring other continents. Perhaps Facebook doesn't have such a strong hold on other continents and Google feels they can capitalize faster there.
In fact, I was invited from Germany. No one that I know of, other than my wife (who was my very first invite), actually recieved an invite from me. My son didn't get one (my second invite). I've been on since July 4th and have tried to invite 47 people (about 3/4 of them are American). Either only one invite went out from me or whoever else got an invite ignored/deleted it... which I highly doubt... because I'm so likable and people want to know me... and I'm a damn handsome man... and smart... ;-)