Mixers are private parties where only the members of one fraternity and one sorority (sometimes 2 fraternities and 2 sororities) attend. They usually have a fun theme.
Here is a link to the Greek Affairs office at Shippensburg:
http://www.ship.edu/%7edeanstu/Greeks/
It should tell you what you need to know and has an email if you want to contact the director of Greek affairs. The page that has a short bio of all the sororities is down (probably to correct its being in all Greek letters, LOL).
Here are some threads you might find helpful (thanks Christin):
http://forums.greekchat.com/gcforums...threadid=34262 (pnm's-why should I be in a sorority?)
http://forums.greekchat.com/gcforums...threadid=35965 (something for pnm's to remember)
http://forums.greekchat.com/gcforums...threadid=30758 (Rush - What to NOT do or say)
Remember to be yourself...you might be able to fake your way into a sorority that is not "you", but you'll never be truly at home there.
There are LOTS of Pennsylvania people on the board so we do know where Shippensburg is
. Good luck!