Larian said we get a team of 4 to keep and take into act 2 (obviously they might have changed their minds).
From act 2 we can put our non main characters in camp and take mercenaries. If they are like dos2 they are blank slates and we can hire them and keep them in the party for as long as we want.
I'm still hoping that some "camp followers" can join us on missions that are core to their stories.
And when we have a keep in BG we can ask other NPCs to stay with us and they can help with research, crafting, trading.
