You and your carpool partners should discuss how to fairly distribute the costs of your commute. You can do this by taking turns as the carpool driver, or by having the driver charge each rider a fare that you all agree on. Remember that there are more expenses than just gasoline. Each rider should be expected to pay for a share of the gasoline, parking, oil, service, and wear and tear. For help refer to following guide from AAA.
AAA Driving Costs Guide