Is the ability to set “Friends of Friends” joining functional, and how does it work?
I don’t believe that it is entirely functional, though @GamersInternatio may be able to tell you. 
I’m not sure honestly, you should be able to join friends though.
Yeah I was wondering this too because I keep getting my friend list full so I want to use an alt to have friends but I’m not sure if people who friend my alt will be able to join my game unless the alt is actually in the game. If that’s the case I’ll have To get another Wii U which I’m not in the price range for. And yall don’t want to up the friend limit so I might be stuck.