I've had some email me with concerns on how a sorority operates, especially the one in my story Tinker Taylor: Spy & Soldier. So here's some information on what a sorority and a fraternity are.
The word sorority simple means 'a house or group of women,' while the word fraternity simply means 'a house or group of men.'
In the US the universities / colleges use the terms for single gender housing organisations that have restricted membership rules and have a management connection to the university / college due to being on the campus grounds so the university / college has some control over them. However, that does not mean that's the only way they can operate.
The sorority in the story has no management connection with the university and it is not on the campus grounds, so it operates under its own rules and under the general meaning of the word.
Now I've explained this, please, no more emails insisting sororities limit access via rushing etc. the way those under university / college control do.