Nowadays, there are several communication avenues that do just that - connect people who want to get to the same event. These include people seeking a ride and people offering a ride. There are websites that can provide a formal carpooling notice-board to help match drivers and passengers.
Remember to promote carpooling through event promotions, any websites and registration materials. You may also want to offer incentives such access to preferred parking spots or even discounted registration / entry fees to encourage carpooling.