How much time between flights did you originally have for your transfer to HKG? If there is any plausible amount of time that you could leave airside (to meet a "friend"), then the TG buggy should take you to immigration if you ask them. If that is not plausible, then you can just ignore the person holding your name on a sign and walk by yourself to immigration.
You probably have enough time to make the Bell bus leaving at 1800 hours, assuming they have room for you. I have never used them, but they do have a good repuation.
But for 2 passengers, I would probably opt for a pre-arranged taxi. The Limo Pattaya has been a reliable choice for me -
http://thelimopattaya.com/
1200 baht for the trip, tolls included. Driver meets you at BKK with your name on a sign - drives directly to your hotel/destination. As opposed to Bell bus which takes you to a bus station, where you transfer to a shared mini-van for the trip to individual hotels.
For the price difference, the pre-arranged taxi makes a lot of sense. You also avoid any stress about trying to catch the last bus.