So its simmilar to Original Sin 1.

The question is then: can you customize your companions or not?
If not that would mean that only if you start the game together you get to make more than one character and the rest is a bunch of premades?

There are some issues with this. One: if that player leaves, his "custom companion", how will that one act?
If the companions are premade: do they have premade personalities? What about characters of other players then?
Two: Sven talked about things like one of your companions beeing a spy for example, could you not easily find out about that by another player taking over that character?