One of the Drive In's that my buddies and I used to go to charged by the person. While other's did try the 'Stuff the extras in the trunk' the theater had people who walked the isles watching for people being smuggled in and most got caught then kicked out without the refund for the people that they did pay for. Needles to say we never tried it.
Which brings to mind this Classic. At a bit under 10 minutes it's a bit long but damn does it bring back the memories--
https://www.youtube.com/watch?v=i674e6f47R0
The drive in outside of the small town I grew up in charged by the car, however they were only about a mile and a half from town and kids would walk out, then climb the fence and hope they could find someone who would let them sit in their car.