Lots of ways to detect temperature with a pic
Since you want to use a 'F84, simplest way to do this (in my opinion) would be to get a **broken link removed** (other makes are available too). It uses a 2 wire serial bus connection to your pic.
I have to agree with the rest about counting the ppl in the room. The 2 sensor-beam idea is the simplest.
On the other hand, does it really matter how many ppl are in the room when you know the temperature? I mean, when you detect 35 degrees, you know its too hot, now mather how many ppl there are ...