It's more or less Act 1, they said that the party could be set after Act 1, so no one would be waiting at the camp while you're traveling once you get past a certain point.
Err, tbh I don't like that. It is something I didlike in another Larian game and with BG as a series, you are usually able to recruit various people and mess around with party composition for the entire game. So if it ends up that everyone else dies because you don't select them, I will be really unhappy with that.
