Press the friend menu in the lower left-hand corner of the screen. Here you can see the people already on your friends list, the people you can add, and also the list of your followers. Tap on the second tab, and select the players you want to become friends with from the list. You can also press the Invite Friends button on the left and use a player's personal code. There you can also find the unique code that you can share with other players so they can add you as a friend. Another thing you can do is add the player you're currently visiting as a friend.