You can play Co-Op by tapping the Battle button on the bottom of the in-game screen. Remember to turn on the location services of your device, as the game will use that to find available sessions.
Playing as a Host
Select the Battle and the difficulty you'd like to play. Then tap the recruit button to start looking for another player. Wait until someone joins your session.
Playing as a Guest
Tap the Join Battles! button and find a session. You'll get two lists. One consists of nearby players, while the other will consist of Friends. Join a session from the list.