Emails are going out seemingly at random to people with a WB ID. If you get one you should sign up if interested, some people are reporting not being able to sign up anymore even with the email, but I'm not seeing any platform info for those denied. Doesn't seem like you can share them either, if you try, it says that email hasn't been invited, though once we get download tokens maybe that will be share-able.
The beta is from August 7-14. Available regions are 360, PS3 (US), and PS3 (Europe).
Maybe add each other if you want to play with friendlies, since it's unlikely your normal friends will get invites.