Two ways of doing this. I register your name, you do mine ala the archaic friend code system. You do this in the friends list tab of the Wii U menu.
Another way, and the much better way is this:
Load up Miiverse. Search for the username, Send a friend request like you would on a PS3. The other person gets a notification and it works much quicker and better. Use this method!
Bassman - RogsR34UK
bowser123 - stubarr7
Fuddle - AvexFuddle
Note for Randy-
Feel free to delete this post and copy it's contents into a new pinned thread of your own please.