I used the Sheraton hotel car service last time I was in HK and here's what I found out:
There's a group of booths for various hotels (the Pen, MO/LMO, Sheraton, IC, etc.) on the right end of the terminal, once you have exited customs. I just showed up there and someone took me down to the lower levels where a car was waiting. Both the airport Sheraton booth person and the driver wore a Sheraton uniform. Not sure if the Pen would have someone right outside customs to meet and greet, but the booths are not too far from where you exit customs.
The driver informed me that the car service is actually provided by a third party firm, which provides the drivers for each hotel to select from. The drivers wear the hotel uniforms and are then considered assigned to each hotel. I had a great experience with the Sheraton service.