You will be able to volunteer on both platforms – Xbox One and PlayStation 4.
With the final release of Injustice 2 scheduled for May 19th for PS4 and XO it’s good to hear that Warner Bros. and developers from NetherRealm Studios decided to schedule a closed beta. You can register on the official site.
The upcoming beta test will be held on both platforms and, as for now, you are not required to use either an Xbox Live or a PlayStation Network account. Simply share your mail and preferred console, and you should received your code in the future. No dates were given for the beta.