I second what others have said...be her FRIEND. Don't change your friendship at all. Just as you would have before, include her when you and other friends are going out for pizza or whatever. She doesn't need words of wisdom. Just do what you would if she were crying over a breakup or an F or bad news from home. You can't fix those things or make her feel OK about them. You can only be a good friend who is there for her during all her ups and downs.
That being said, while you're entitled to all the joy and fun of your new group, I would refrain from rubbing your roommate's face in it. In other words, have a great time with your new sisters, but I think you should skip the giant sorority letters on your (shared) dorm room door. You don't need to hide your affiliation, but this is not the right time to cover your whole side of the room with XYZ gear.
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CurvyLadyLips