Well, I can't speak for being in a sorority in college, but I was in a Fraternity. We had a chapter house which was quite large, but there were too many members for them all to have rooms there. Some of the guys lived there while the rest had apartments or still lived in the dorms.
We were selected for the fraternity during rush week. This is a period of one week before classes begin when incoming freshmen come a week early. They tour the various fraternity houses and then the fraternity members vote on who they want to make offers to. Fraternity members then come to the freshmen's dorm room and give them the card or letter with the invitation on it. Some freshmen, particularly those who are attractive or seem to have a lot to offer are given offers by multiple fraternities and have to choose to accept one.
Others are lucky to get one offer and some get none.
In my fraternity, it was usually the upper classmen who got the rooms in the chapter house since most of the freshmen had already committed to the dorms before being selected to a specific fraternity.
I don't know if that helps any. You might try these links:
http://en.wikipedia.org/wiki/Fraternities_and_sororities and for a list of fraternities and sororities:
http://en.wikipedia.org/wiki/Category:Fraternities_and_sororities
also...here's a quote from the wikipedia article about fraternity/sorority chapter houses:
Fraternity and sorority houses range in size from three to twenty bedrooms or more. They can usually be identified by large Greek letters or flags on the front of the house. The larger houses generally have a large meeting room and/or dining room, commercial kitchen and study room. There is usually a lounge of some sort, access to which is often restricted to fully initiated members. Fraternities and sororities will also often maintain a chapter room, to which only initiates may ever be admitted and even whose existence may be kept secret. The walls of the house may be decorated with pictures of past chapter events, awards and trophies, decorative (or historic) paddles, or composite photos of members from past years.
In some fraternities or sororities, only the representatives live in the houses while in others the entire fraternity or sorority may live in the house. Other, larger fraternities or sororities may have more than one house to house all of its members.
Hopes that helps!
