It's kind of an important thing.. The short version of it is that some of the others convinced Joyce to go to a party at the college. Sarah protested that it was a bad idea, but they went anyway. Joyce met a guy there who pretended to be nice and religious, but was just looking to get her alone so he could rape her. He slipped her a roofie at one point, and while she was fuzzing out, she hit him with the glass she was holding, cutting his face and her hand. That's how blood got on the orange sweater. Then Sarah showed up with a baseball bat and slugged the guy. During all the confusion, he slipped away and the girls got Joyce back to her room. Despite others urging her to report it, she just didn't want to think about it and eventually they relented. Joyce has been stewing about it ever since. The guy that Joyce kept seeing everyone as a few weeks back when she ran from gender studies to find Ethan was the attempted rapist. And probably a serial rapist because he's apparently been known to cruise college parties like that.