For now there is a workaround...
Launch the game(on minimum gfx settings to save the pc struggling) and start a multiplayer lobby. Tab out of the game and launch the game via the main exe file in the steam local directory and join the game via lan. Do this again until you have a full party.
Launch the campaign and switch between instances of the game making the party how you like. Click venture forth with all 4 instances with the game. As soon as the cinematic starts save the game on the 'host', then quit as players on the other 3.
And there you have a custom party.(at this point you can turn the gfx back up)
