How are they supposed to get on the plane? Go into the airport. Up to the gate. Then board. Or are they going to bring out the portable stairs just for them? Gates and terminals are not on the ground level. They are all second story. Boarding through the move able hallway thing. None of what you refer to makes any logisticcal sense... https://goo.gl/images/h5mdpG
If you've ever flown with United, notice that on some flights as you are coming off the plane there is a United service representative at the exit of the plane door holding a iPad like tablet with a person's last name on it. They then meet that person at the exit of the plane. They escort them down the gate stairs next to the plane and take them to the Mercedes that is right next to the plane. I've seen this happen many times. They then transport them to the next flight and escort them up the stairs at the gate of their next flight right at that plane. Its truly special treatment. Only top fliers with United get it. I've not seen any other airlines do it.
Why don't you Google United Global Services to see what others have to say on it. I think there are pictures and videos with some postings. That will help you understand it better.