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.
You can also remove a friend by tapping the recycle bin button.
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.