How do I add a Friend in Game Center? (iOS 9 and below only)

This FAQ describes how to add a Friend in Game Center.

Note: These steps no longer work in iOS 10 and higher since Apple has removed the Game Center app.

Steps

1. Sign in to Game Center if you haven't already done so.
2. Launch the Game Center app:
   
   
3. Follow the screenshots:
   
   
   
   
   
   
   
4. Your friend will get a notification that you've invited him/her. When they accept the invitation, you'll see them in your friends list: