To add a friend, tap the Friends button in the main menu screen.
From here you can see a list of all your current friends and clan members.
Tap the "+" button in order to search for a friend's username and add them to your friend list.
Type your friend's username and tap the search button.
Press the "+" button on the correct account in the list.
Your friend can find their unique account number in the settings menu.
You can also search for the exact account name and number to instantly find the correct account you're searching for.
You can now also add friends by joining the same group in the main lobby, and pressing their profile when they join the game.
In order for you to delete a friend from your Friends list, you first need to go to your Friends list and find that friend.
Click on the friend's username and you will get the option to either Block, Report or Delete that friend.