No, I wouldn't since it's dishonest. Plus I wouldn't want to stick someone in a hot trunk scrunched up to save a few bucks. I do have to admit that my girlfriends and I did this one time when we were in college though

Bad aliadam!
Our local drive ins have charged per person for as long as I can remember. We used to take cars full of kids to the drive in when my kids were younger and we always had to pay for every one of them.