Actually, there are two ways to add friends.

First:
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.

Second:
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.