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.
I don't have any better evidence than the 5ish seconds of trailer. It's just a hunch. I have two more hunches-- first, Sven has stated that the origin characters you don't select become unavailable (I'm guessing because they get mind controlled by Absolute or become mind flayers, or whatever), but that there would be new members afterwards. Why force a selection if you were just going to repopulate your camp with more squadies later anyway? It only makes sense if they were trying to limit story permutations on the origin characters. Subsequent characters could be relatively simple narratively. Second, Minsc is a beloved character and he is voiced by a D&D celebrity, how can they expect any of their origin characters to compete? Lazel or Karlach would have a real hard time making the cut over Misnc as your front-liner? It just makes sense for a slot to open for either of Minsc or Jaheira.