according to the dev just like in D:OS2 it will be possible only in act1, and after that we will have to choose a permanent party.
Source, please?
Quote
Is your party permanent or can you change members out throughout the adventure? Recruited companions will be at camp when not in the adventuring party, and can be swapped in and out at camp. After the first act however you are going to have to commit, also just like in real life.