People that have datamined the game have found companion strings for 8 more characters.
It would be a very poor decision to limit new companions to just the first act.
In fact in real life, nobody has to commit to companions, and you always meet new people later in life. You don't have to commit to anything actually.
That reasoning seems very weak, and makes it seem as though the people that are making this game have never once in their whole lives stepped outside of their mother's basements.
Yeah, I've been wondering at how the whole "commit to party by end of Act I" fits together with the fact that we probably only have at most half of the total companions available now. I mean they could probably fit a couple more into the EA area without crowding it too much, and I suppose the rest would have to be added on the next leg of the main quest since that seems like it's the end of Act I.
As for the real life thing, I assumed they meant in a roleplaying group because otherwise yes, you're quite right, that's not how it works at all
