The tactic of "zerging" stems from games like EverQuest, where a raiding guild would use many many more people than what is necessary to complete the task at hand (killing a big, evil, mean guy.)
So "riding a zerg" would just be to go along with the flow, get in a squad that's already well set up, and kind of chill out in the middle of a giant attack/hack on a base. This way you get shared XP (from the squad) and base cap xp (from the base cap that is accomplished after you kick ass in the base SOI), without really doing much work yourself.
Hope that helped
