1) Tap on 'Friends' button on the Homescreen.

2) Tap on 'Add Friend' button.

3) Search your friend by their username.  

4) Tap on the Search result.

5) Friend request is sent.

You can also add the friend from the match end screen by tapping on the '+' icon and selecting 'ADD FRIEND'.

The 'ADD FRIEND' option is also available if you visit a player's profile through chat.