I bet you can get 90% accuracy by just estimating the number of departures based on the flight times of everyone entering the club, because that is validated at check-in. I'd probably assume passengers leave the club, on average, 25 minutes prior to departure.
You can also use thermal or other types of imaging (e.g., my comments about the parking garage above).
They don't need exact counts.
It would seem to be a good use of capital given the negative publicity of long lines at the clubs, if they actually get more people in when there's still plenty of room (i.e., it's not clear how often this is an issue, other than various anecdotal reports.)