Have they said they will lock the party? I got no real indication or feeling that it was the case when I finished the EA. But it does seem like you can't have all the characters with you if you make certain choices.
Pretty sure Wyll will not take kindly to you joining the Goblins, and Astarion get's very crabby if you get angry at him for being a vampire.
All in all it doesn't seem to me that the way the game is set up is going to force a locked party, in fact it seems like the opposite. If they force the party, they cannot have party members leave the party, or else you are stuck with just 3 members.
