How to Modify the Bidirectional Visitor Counter Circuit?
This is super easy to do.  All you need is a digital input that goes high when a new person is detected, and is low when an already detected person is detected again.  From an electronics perspective, it's easy.  I see PIB2 is open on the micro, so why not use this?  A little fiddling with the code to change the logic, and you're good to go.  

The difficulty is, how do you recognize a person?  Biometrics is an active research field.  Facial recognition is possible, and we're starting to see relatively accurate results from new implementations.  If you were to go this route, I don't think a microprocessor's going to cut it, anymore.  I'd recommend a mid-grade computer controlling a camera to take a picture of each person who enters the room.  A good starting point for getting facial recognition working is OpenCV.  I'll gloss over the details since there's a number of algorithms to do it, but in the end, all you need is that digital out running to PIB2, and you're gravy.