There are two ways to play Silksong using the multiplayer mod: Steam: If both players have the Steam version of the game, simply press F5 to enable the Silkless mode and invite your friend directly ...

If you want to bring along a friend for the ride, there's also a "multiplayer co-op" mod for Silksong you can install on PC right now. I put "multiplayer co-op" in quotes because it's not true co-op.