you need to use the friends app and get the person's friend code

like my friend code is 4051–9155–7005

wiiu you hit the homebutton , go to friends list and choose "register friend" you can then send a friend request by typing that persons nintendo id name

on the wii u you also in the friends list can go to "users last played" to see a list of everyone you played online with and send them a friend request or block

Go to your friends tab and enter the user's friend code, note that both users must exchange their code to actually be friends