A couple of points-
1. You as a potential new member are NOT allowed to speak to the sorority members before recruitment. This is so that recruitment stays fair.
2. Below, I found a link to a girl's rush story. She rushed at UNC-Chapel Hill not only as a sophmore but she also joined a sorority and depledged the year before. This leads me to believe that UNC is not highly competitive meaning they only take freshmen. However, being a junior is a different story. No one can stop you from rushing, but keep in mind that it may hinder somewhat from the opportunities that freshmen and sophmores have.
UNC Rush
3. You are not expected to know everything about each sorority on your campus. But it DEFINITELY helps. You want to ask the girls a lot of imformed questions during rush. You mentioned that you do not know everything about them. Well find out! Research them. If you want to get recs, you should get them for as many chapters on your campus as possible. Don't just get it for the ones that you are most interested in. You have not been through rush yet, so your opinion could change. Then you will regret not getting a rec for that particular chapter.
Here is the link to UNC sororities. Almost every sorority listed has their own personal website. RESEARCH!
UNC Sororities
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