Image processing on a surveillance image
Thanks for sharing your experience. It's a bit off-topic, but my my first advice for a job interview would be to check the company's technology and products. This usually helps a lot in finding answers.

In this general case of crowd surveillance in a closed place, I would check the following items first:


get a reference image of the empty location in order to be able to detect people's location (even inaccurate) by background subtraction. This gives a rough estimate of the queue length;
check if the camera is facing the line, in which case I would run a face detector to get an approximate count of people in line. This provides a second estimate;
if necessary, learn (from the detected faces) the skin color in the image, then again segment the image based on skin color.


There are more advanced techniques available, but I think it would be overkill here.