Every player has a friend code: open the Friends page to see yours, and tap Copy to share it. To add a friend, tap Add Friend and enter their code. They get a friend request on their Friends page, and you become friends as soon as they tap Accept. Until then your request shows as Request sent, and you can Cancel it.
If you get a request from someone you don't want as a friend, tap Decline; they are not told. A request nobody answers expires after 30 days. You can have up to 100 friends.