We found out earlier today that Super Mario Maker 2 will not allow you to play online with your friends, you can only play with random players. If you want to team up with friends you will need to play locally. The news was first reported by NWR and GameXplain has chased it up today with Nintendo of America who confirmed to them that this is indeed the case.
“This is true. As the game is now, you cannot create friend lobbies online. You can only play online with randoms. You can still get friends together and play with local play, and you’re free to play with four players on one Nintendo Switch system. You can also still challenge your friends to your creations and take on their designs through Course IDs and having them follow your Maker profile.”Nintendo