Parking a car full of luggage near Paris for a half-day trip into town. Not enough time. It will take you a half day to find the police station and make your report after the cars are broken into and the suitcases removed
RER Station is at CDG, and by the way, so is the TGV Train station that would take you and your 11 companions to Lyon in 2hrs and for something like 800€ round-trip for the lot of you (+/- 200€ depending on train time, and group discount). I would take the TGV and forget the cars all together. If you really need a car, rent one locally in Lyon.