If the car pool passenger must travel to a central location for pick up, the trip to/from that location is considered the commute, the same as if they had to go to the office to then go to the worksite from there. Therefore, the time after that--after they get to the collection point or complete their commute--is work time. The can't make the employer pay more by deliberately showing up earlier than they need to, so as a practical matter, the clock starts for them when they get into the driver's car to be driven to the worksite, the same way that it is work time for an employee to be driven from his/her office to a work location.