To join friends online in Mario Kart World, you need to do the following: Make sure all players have a Nintendo Switch Online Membership. You can't play online games without them. One player goes to the main menu of Mario Kart World and selects "Online Play".

Using local split screen, you can play any of the single-player game modes with your friends, except Free Roam. This means you can take part in any Grand Prix cup, play Knockout Tour mode, race in regular VS Races, and knock heads in Battles.

It might not have all the features of solo, but you can play Free Roam splitscreen in Mario Kart World.