Nintendo Join Error

When joining a person's game, it does not let me. An error pops-up and says, "unable to connect please restart client." I don't know how to fix it or if anyone has had this problem. I don't know how to restart "client" but if any knows please let me know. It's not a problem on my game, it's when I join someone else's. I can join perfectly fine when joining a server.   Thanks.