Shrooms, or psychadelic mushrooms, work by food poisoning the body, and the body fighting it off. You do not actually trip off of the shroom, but off of the chemicals your body produces in responce to the shrooms. These chemicals are not reabsorbed in to the body, but passed out. This means that is someone is shrooming, you can drink their emitted bodily fluids (piss, semen, etc) and you will also trip.
I can't say I've ever wanted to trip badly enough to want to test this method, but if sausage_boy want's people to "share", he might want to make sure how they plan on "sharing" first.