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.
Very good to know, but how does one access this variable to change its value? Is that a complicated process, especially in the context of people like me who are not at all versed in coding?