If you're using a 3ds there is sadly no way, on a Wii U Pretendo is still working on it

i comes up with an error when i try to get online

Friend joining on worlds and minigames work, just not minigame public lobbies, as it requires a matchmaking system to be developed.

Thank you for letting us know, Robin. I had no idea! I thought it didn't work at all! Hopefully public lobbies will work soon.