We can add the friends in the game by tapping on friend icon which contains the option "Add Friend".
Here you can add new friends by searching the manager’s ID and sending invitations. And by click “Friend Request”, You can see pending friend requests that have been sent to you by other players.
Tip: Remember to manage your friend list to have active friends as many as possible (up to 50) and collect more AP and Friends’ Pick.
You can also challenge your friend by click "Invite" and your friend will see the game replay in "Messages">>"Friendly Records"