The philosphere stones are very similar to magic mushrooms, the main psychoactive compounds are psilocin and psilocybin, although there are thought to be other alkaloids present which are thought to influence a trip, as with other magic mushrooms. So if you want to trip for a good couple of hours then split the full 15g with your friend, personally i would take around 5g or more made into a tea. Hope this helps, and enjoy:D
"He who makes a beast of himself gets rid of the pain of being a man"-Hunter S. Thompson.