There are 2 ways to add players as friends.

If you have an ongoing or a finished game against the player you want to be friends with, open the match, click on the opponent's avatar and press on the "+" icon on the left-hand side of the avatar.

Another way is to go to the 'Social' tab and search for the player. Click on the player's avatar and then press on the "+" icon.

To delete a friend go to the ‘Social’ tab, click on the avatar of the player you want to unfriend and then press on the "-" icon on the left-hand side of the avatar.