I think the resident GrandCentral users have run out of invites; I put my request in 3 days ago, and haven't gotten one.
But then again, I'm a jerk
I think they're just being stingy . New users get 10 invites and I gave out 5 to people in this thread after sending the others to my family and friends. So at that there should have been 50 available. Hmmm.