I have a feeling you are party size locked until your "infected" group is picked and then you can get non-infected to join (Minsc and Jaheira) and the party size increases. Look at 1:20 in the game awards trailer. There is Jaheira, I *think* 4 Tavs, a bunch of flaming fist soldiers (2 archers in back by the door, 4 knights up front), and a few others in front I can't explain. It's possible Jaheira isn't in the party, kinda like the fight with Halsin, but I like to hope.