6
There isn't a consistent answer here, since it varies wildly from game to game. Most games with online play will have some way to interact with friends, most often with private lobbies that only your friends can join or the ability to join a friend in an existing public lobby, but one thing that's consistent throughout almost all games is that you can't send out a call to friends to join you; If you want to deliberately organise some gameplay with your friends, you'll almost certainly need some alternate communication method, since Nintendo doesn't offer any.