I remember reading a post from Larian that Act 1 will be the only Act where you have all companions and that you will have to choose just like in Divinity OS 2 who you take.
"Will companions be interchangeable during long rest? Yes, at the start of your adventure your recruited companions will be at camp when not in the adventuring party, and can be swapped in and out at camp. Just like friends in real life! After the first act however you are going to have to commit, also just like in real life."