To follow a friend, you must know their username. Tap on the Leaderboards icon at the bottom of the main page, then tap the Friends tab on the Leaderboards screen. Tap Friends Request and type in the username of the person you’d like to friend and tap search. If you see your friend listed in the search results, tap on Friend Request. If you accidentally tap on the wrong user, you'll see the option to cancel your request.
If you don't get results, please double-check the username you’re searching for and retry.
To unfriend someone, just tap Manage Friends, find the friend you wish to remove and tap the "Remove Friend" button. Confirm that you do want to remove the friend and you're all set!