Here is the Student Union website with the Student Group directory.
Student Group Directory | Student Union
Click 'List All Groups,' and there you should find a pretty comprehensive list of all student organizations.
Just a sampling of what you can participate in (a very small sampling!)
Volunteering:
Campus YMCA: Host parties for children staying in the hospital; tutor kids in disadvantaged areas; be a big brother or sister mentor to a younger kid; go on Alternative Spring Break trips to fix houses in a rural community; educate the community about AIDs; put on science experiments for children at local junior highs and high schools...and of course, MORE.
Trivia Team:
Have you got BUZZER skills?
KWUR:
Be a radio host. Select the music, have your own program.
Spires:
An inter-collegiate literary and art magazine founded at WashU (submissions from colleges around the nation)
The Troll Club:
"We are trolls. Help us live with it. For the promotion of troll culture."
WU Cypher:
Calling all bboys and bgirls ('breakdancing' group)!
WUnderground:
WashU's satire and comedy student publication
WUSLAM:
For the slam poets.
As you can see from the list-- there are too many groups to list. Check them out for yourself!!!