Once you enter the game mode, you will see an icon in the top right corner of your screen.
Once you press X you will see the Available Hosts screen. You can then select the host you would like to join.
This will allow you to play an 8 player game with 4 players per console.
You can connect only 2 Nintendo Switch Consoles together at one time, you can play 2 v 2 or 1 v 3.