sm64coopdx

What you need: A ROM of Super Mario 64 dumped from your own copy of the game. See the project's own page for the versions it supports.

sm64coopdx, short for Super Mario 64 Coop Deluxe, is a native PC port of Super Mario 64 built from the game’s decompilation, focused on online multiplayer.

Up to 16 players can explore the game together, each as their own character, with Lua mod support, custom characters, levels and game modes.

The download contains no Nintendo data. It asks for a ROM of the US version of the game, dumped from your own copy.