Coarse measures:
- Force switches on the waiting room seat legs (empty|not).
- Humidity in the waiting room exhaust air.
- Force switch doormats under the queue.
- Vending machine sale rate.
- Take a number.
- Direct observation by the manager who damn well should be watching.
How about a camera and a frame capture card for a computer. The the number of people (approx)is some function of the picture area that's changed from the background.