Actually, there are two ways to add friends.
1) Go to the Menu - Friends;
2) Tap on the «Enter the player's name» field;
3) Enter the exact nickname of the person you want to add and press ok.
Once the person accepts your friend request, you'll see them on your Friend list.
1) When the match ends, go to the Scoreboard section;
2) Press the button with a plus sign near the player you would like to add as a friend.
To delete a friend, go to the Friend list, tap on the name of the person you want to delete and press the minus button.