Multiplayer support for the MacOS, Linux, and Android versions of the game will come in a later version of multiplayer.
Viewing post in Baldi's Fun New School Ultimate comments
Yes, that will be one of the options available when you are setting up a multiplayer game. You'll be able to either create a game or join one with the name you entered that has the same maximum amount of students (aka players) you specified, or join a random game (or "random lobby" as you call it) that is not full yet.