Just a heads up for those that want to try this before it’s an official feature- you can make a custom party by starting a LAN game with yourself.
Open the game directory (browse local files on Steam), open the bin folder, launch the game from the exe as admin. You can do this 4 times so you are running the game in multiple windows. Start a LAN game with one and join with the others. Create a character in each, venture forth, and close all of the BG3 windows except the host.
I don't suppose that something like this is possible on Stadia, is it?
