I'm like 65% sure they checked my boarding pass before getting on the airport express 2 weeks ago. I was able to get off the train without a BP, but you can't really go anywhere, and with the security, if you loitered, you'd get questioned at minimum. Please note that even if you get a car service, I don't think they'll let you wait outside the terminal and they won't let you in the terminal.
Unless the people you are meeting are elderly, unaccompanied young children, or infirm, I strong recommend arraigning to meet them at an airport express station elsewhere along the line. It is insanely easy to use.