washu's a cappella scene is quite good and quite large for a school the size of washU. there are lots of really great groups, and there is a great sense of community among them.
Mosaic Whispers, Amateurs, and After Dark are the 3 co-ed groups, all performing variations on the "rock-pop-contemporary" theme. I know that Whispers is the oldest of the three and has appeared on like 4 national compilations, tours twice a year, and has released 8 CDs (their new one just came out). All 3 groups are great, though, and each has a unique personality and reputation. The oldest group is the all male pikers, and a newer guys group, the stereotypes, also sing on campus. there is an all girls group too, called the greenleafs. There are other niche groups, who concentrate on specific types of repertoire or audiences. These include staam (coed jewish a cappella), more fools than wise (coed madrigals), deliverance (coed christian a cappella), and the aristocats (coed disney a cappella).
a cappella rush happens every fall (and sometimes in january too), and all or most of the groups participate. it's run by an internal organization called ACAC (sugroups.wustl.edu/acac). getting into some of the groups can be quite difficult (over 100 auditionees for usually 4-6 spots). It's definetly worth a try for anyone though, even if you dont think you can sing! At the beginning of the semester, the groups are EVERYWHERE, and there are lots of events and performances to help you get to know the groups.
Each group hosts a concert or two on campus each year and does lots of performing on and off campus. These concerts, like Splash of Color (the whispers show) in March are well attented by the student body. Some groups also go on tour and record CDs. Most of the groups rehearse like 7 hours a week and have a gig or two a week. Some groups chose to compete in ICCA as well, but some, choose not to on occasion as well.
Try out if you come here...its amazing, and everyone I've known in the a cappella community has loved it too.
WashU's All A Cappella Auditions Committee: http://acac.wustl.edu
Collegiate A Cappella Directory: http://www.collegiate-acappella.com
Recorded A Cappella Review Board: http://www.rarb.org
Important Note: I know more about Whispers because I am in the group, and am also a former member of ACAC.