In order to add a user to your list of friends go to Menu>Friends and write their Facebook name (if their account is linked to Facebook) or their user name in the search bar. Once you find them, tap on "Follow". That way, you'll be able to play together and, if the user's account is linked to Facebook, they will appear in the weekly ranking.
If you want to remove a user from your friends, you must first find them through Menu > Friends, by their username or Facebook name. Once you select it, next to its profile you will see the legend "Following"; If you tap on it, it will give you the option to stop following him. The user will be then removed from your friends.
In case the user continues in your friends list after removing it, we recommend that you force stop the app and then open it back again. The user should not appear on your list anymore.