you have to be invited although it seems that google is giving away invitations as fast as they can be passed out. i had 6, gave away 4 and now im back to 6 again. i was under the impression that they were going public with this by now...
It's still in the Beta testing stage. They're controlling the number of users. They randomly give out invites each time they are ready to expand. They do seem to be giving them away quite liberally now... maybe they're close to opening it to the public.
