Two alternatives you could also think upon:
1/You could use a laser and photo-resistor pair. On-Off-On signal will be given out and you need to judge whether a person has walked through on basis on each of these edge and level duration.
Laser since its directional .
2/If its a managed crowd , you could have a manual switch itself, which is to be pressed by each person who enters. A beep response (and the display) you have could be used as an acknowledgement.
3/ The IR-pair on opposite sides too is good but you will require additional circuitry for tx, which is not required if you use 1/
Regards,
SAV