The closed beta is for the multiplayer portion of the game. Players will have access to six maps which which to play against other players in solo or team matches. They may even play co-op against the computer AI. According to the press release, "Players also get access to the first 40 levels of progression, and the ability to customize their loadouts to suit their play style using the all new Army Customizer."
To join the closed beta via Facebook, gamers can Like the official game page.
For the latest hands-on preview, written a few days ago, check out Joe Robinson's article.