I know at my school, you have to pay a nice amount of money to join a sorority. As for what they do, it varied from house to house and from school to school. The best way to find out would be to contact the head of one.
I personally have never been in one, but I know of people who have joined Greek houses and they loved them. It all depends on what you like and what not.
"How do we tell the good guys from the bad guys?"
"If they're shooting at you, they're bad!"