I'm a current student at WashU and a member of a sorority.
There are 7 sororities currently at WashU, and 1 additional sorority will be colonizing in spring 2013. The sororities are:
Alpha Epsilon Phi
Alpha Omicron Pi
Kappa Kappa Gamma
Pi Beta Phi
and we'll be adding Kappa Delta next year.
Each sorority has a different reputation, but for every rule there are many exceptions, so I won't get into the specific reputations. There are wonderful girls in all 7 (soon to be 8!).
The sororities each have a suite in the Women's Building, and hold weekly chapters Sunday nights in addition to several social or philanthropic events during the week. Each sorority has a specific philanthropy associated with them - I don't know all of them off the top of my head.
The great thing about WashU sororities is that they're really what you make of them, time-wise. I am involved in a lot of different things and have found it easy to set sorority things aside when my other commitments get busy, but I know plenty of other girls who make their sorority their life.
Your GPA will certainly not fail! If anything, it might improve - sororities have a lot of internal help for studying and resources on campus, and you'll get in trouble if your GPA drops too low.
I would never have imagined myself a sorority girl, but have seriously enjoyed my experience at WashU. (I won't tell you which sorority I'm in - Panhellenism and all that! You should check out the WashU Panhellenic website for more information about each sorority - Washington University Interfraternity Council
[I know this is an old post, but I figured there should be information about WashU sororities on here somewhere!]