I recall having seen a "human detector" on Sparkfun recently. Maybe that is suitable for your project.
Another option: break beam sensors in the doorways (to see if anyone passes), plus motion detectors inside the room.
The break beam sensor tells you if someone is in the doorway (going in, going out, or just standing there trying to remember which way they were going - doesn't matter).
Then you have a motion detector in the room. If you detect motion in the room the break beam sensor is not triggered any more, someone is in the room. Even if you stop detecting motion, that someone is still in the room. Only if your break beam sensor is triggered again (someone in the doorway) you have to check for motion.
If there could be more than one person in the room that still won't work very reliably though - the remaining person may be so motionless that the motion detector thinks there's no-one left.