You can retake dismissed party members in BG2. They either return to a set location (which they tell you about), or stay on that spot.
The only time you are stuck with the party is Chapter 4/5 (Spellhold/Underdark) and 7 (the final).
Yes I know. I have played BG2 before. Come on, you know what I mean guys.
Your original post made it sound like you were stuck with the same companions the moment the party was filled and couldn't switch them.