You may get some clues from the types of events the sorority is holding. If they're holding a picnic in the park, you may be able to get away with something more casual; if they're doing a formal tea, get dressed up a bit. The informal rush information might even include suggestions as to how dressed up to get.
I would avoid cutoffs, t-shirts with printing (especially if alcohol or smoking related), anything that looks like you're going clubbing afterward, and anything damaged. But you probably don't need the latest Lilly dress for informal.
Good luck!
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