I think by the end of Act 1, we have to "lock in" our party. I'm not sure if this means anyone you don't recruit will forever be unrecruitable or if it means any Companions not of your 3 gets dismissed. However DOS2 did this, might shed some light.
If party members do get dismissed, and they're still alive without you, it would be pretty cool to encounter them later in the game going it alone. It really does feel though that most of these characters don't fare well without the party...
