Battlegrounds. First off, the email confirms the game will drop in North America on PSN (and assumedly XBLA) in the same week as Europe, on April 5. Entry to the 'grounds will run shoppers $9.99 (800 Microsoft Points).
However, if you'd like to eschew that fee, you can simply enroll in the PlayStation Plus program before March 25 -- doing so will allow you to grab the game for free and keep it forever. It's unclear whether this deal's available for everyone, or just the people who received the aforementioned invitational email; we can't imagine Sony left too many people off that mailing list, though. They rarely care about exclusivity when it's money time.