The FAQ about private beta in the "Good news everyone!" email states
Q: Who can access the site during the private beta?
A: Only the 388 people who committed to it.
Q: What else?
A: The minute the public beta opens, invite your friends. Spread the word via Twitter, blogs, and email far and wide. Hang out on the site answering incoming questions during the first few hours so that newcomers understand what a great experience a working, popular Stack Exchange site can be.
(Presumably, "invite" here means "encourage your friends to join" rather than "send them an invitation")
But the private beta website has
Invite Fellow Experts
email a private beta invite to:
Perhaps inviting experts to the private beta is a new feature, and the FAQ about private beta hasn't been updated.