If you wish to start a game with a friend, tap on "New Game". After you do that, a screen will be displayed, in which you will have to enter their username or the email address they used to create their account. After you do that, you'll have to tap on their profile picture to start the game.
Remember that you'll be able to invite friends that don't have a game account by tapping on 'Invite friends' in the 'Games' tab.
If you are part of a team, you'll also be able to play the Tower Duel mode with your teammates. If you wish to know more about how to play with your teammates, please visit this article: How to play a Tower Duel with my teammates?