I am a newbie to the club and have not been able to make it out to any of the Tuesday night rides... I still have an opinion though and I'll share it for free, after all that's probably what it's worth (nothing haha).
I think SmarRiders suggestion is a good one... From the sounds of it there seems to be a good amount of people there each Tuesday night riding (30+ riders?). Why not have someone bring a sign-up sheet and simply ask for volunteers that are willing to give up one night of riding in order to stay back and make sure nothing foul is happening around the cars. It would probably work out to each person having to give up one night of riding no more than once or twice in a calendar year. If you keep a tally of who has volunteered to stay back, those who haven't might be more likely to volunteer as well. I don't think it's necessary to have people signing up more than a few weeks in advance... Rather, try and make sure each week that the next few weeks are accounted for, and then make a posting a couple days before the ride to make sure the volunteer remembers that this is their week to watch over the cars.