If you have an iPhone or iPad, you've got Game Center. Once you log in, you're ready to start playing games with other opponents. You can share your Game Center ID with others and receive invites to become friends. From there you can view each other's achievements and challenge each other to games. If you aren't sure how to accept a Game Center friend request, here's how.
While you can always accept friend requests as soon as they come in by tapping on the notification, you can also accept multiple invites at once or accept individual ones at your own liesure.
That's it. The contacts will either be added to your Game Center friends list or rejected based on which option you chose.