The variable is called DB_Origins_MaxPartySize(1) – and normally has a value of 4. You can change it up to 8. That's literally how easy it is, you don't need modders to work on this. Which means Larian has built the engine and the UI to handle parties of that size. I understand it's so you can have followers etc too, but basically larger parties are effectively already in the game, it just needs to be enabled.
It would be nice if encounters scaled based on party size (in reverse too for people that want to run a smaller party). Hopefully that is something they are working on, assume we will be getting difficulty levels at some point anyway.