There's no friend search function so I can put his number in and add him.
This is how I did it lol. First you have to know your friend's IGN, then ask him to spam at the public chat channel. When you see his name quickly tap on it and you will see the "Add friend" option. To have him add you back it's much easier just private message him from your friend list.
The hard part is to spot your friend's message in the 10000000 disturbing spams.