Been Mawed | Cool Marine/Warthog Glitch (double plus fun)
How did they get those guys up there?
Now that's a very good question - glad you asked. They call it the 'Alley-oop' manouver:
The movie below shows the actual 'teleporting' of a marine, and helps to show where you need to leave from. (You're backing all the way down, so position can be tricky.) Hearty thanks to Frogblast for the actual filming.