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.
That...is an interesting theory. If that is truly what Larian is doing for party size, allowing for beyond 4 after dealing with the initial selection, then I could be fine with this.
But, there is no indication or hint that this will be possible, beyond having random NPCs that temporarily joining you for some side mission or fight.
