You aren't allowed on the ramp in the back, unless you need to help. You stand around near the ECV, the driver loads the ECV(s), they load you through the front. He will direct you to cut the line, if you need to, if the others are crowding.
I haven't seen a limit on the number of people, though I guess I could imagine it happening. 6 should be no problem.
Edit: I am assuming the old style Disney transport buses. They are using some new stuff now here and there, including some non-accessible Mears buses, and some buses like the old ME.