When you first load into the Player vs Player Arena in Injustice 2, you will be placed into an Arena with players who began playing the game around the same time as you and have had a similar opportunity to progress their collection & characters. When you battle in the PvP Arena you will only compete against and be matched up with players in the same Arena group as you. Rankings & rewards will also be specific to your Arena group.
It is not currently possible to change Arena Groups through any means.
Please keep an eye on your in-game inbox for official information and updates from the Injustice 2 team about the Arena and matchmaking!