Correct, the powers only activate after they're in! Ideally, they wouldn't activate until after people have discovered that their Pokemon won't attack the boss, but that's going a little too far in micromanaging characters' actions, we think.

And yes, three people must open the door, but they can let through as many as they want while they hold it! A perfectly valid form of plot participation would be "professional door-holder", for example — keeping the passage open so anybody who wants to can get through. :|b