I’m aware of the method for having someone in a room, but how do I have them enter the room? For the sake of discussion, let’s say Bob walks in at the start of play.
For the specific situation you mention, the best way is just:
When play begins:
say "Bob walks into the room.";
now Bob is in the Atrium.
So I guess first I’d have to define Bob without mentioning a room.
Or you can have him start somewhere:
Bob is a man in the kitchen.
When play begins:
Try Bob going south [or whatever direction moves him into the Atrium].
The nice thing about that approach is it takes advantage of Inform’s built in notification rules for NPCs moving around; “now Bob is in the Atrium” basically teleports him so you’d probably want to write your own report so the player knows what’s happened.
Or that.
Of course I defined the wrong room first, so it’s now a matter of shuffling text around or somehow changing the starting room. Good times.
The thing to remember about Inform is that actions only matter insofar as the player sees them. If it’s easier to accomplish something by just faking the effects and narrating it yourself? Go for it.
Mike’s solution, of “try Bob going south”, is ideal when you don’t have all of Bob’s movements planned out in advance (maybe he moves randomly and you have to catch him), or when it’s important that his movement triggers the normal action processing rules (maybe you have to lead him across a dangerous bridge where the troll will stop him unless he gives it a treasure). But the descriptions are deliberately rather bland, since they’re meant to apply any time anyone enters or leaves a room.
If you want a grand entrance, there’s nothing wrong with bypassing the action handling entirely!
The player wears some itchy trousers.
Every turn when the player is not wearing the trousers and the King is offstage:
say "A voice rings out from behind you. 'Please rise in the presence of His Majesty, King Rupert III of Michigan!' You whirl around to see—";
now King Rupert III of Michigan is in the Atrium.
No going action is triggered here, but the player doesn’t have to know that!