Until now, Closed Beta for AC2 has been open to AC1 players only. On September 6th, that will change as 1000 players will be choosen to participate in the Open Beta for AC2. The one catch is you must have a Micro$oft Passport in order to sign up. Good ole MS. Ever wonder what the passport is really for? ;->
If you are interested, you can get the full scoop here, as well as sign up.