In answer to your question, that's definitely not illegal...unless it's untrue. If it's true, then the First Amendment could protect the group creator's right to publish that information. If it's not, then it becomes slander which is a subcategory of defamation and is punishable by law. The guy could take it to trial and even sue for damages if he lost his job because of the accusation or something similar. However, he'd have to provide concrete evidence that he's innocent in order to make the charge stick. Even so, if the group creator says she really was raped and honestly thought it was him she could still get off on a technicality. He would have to somehow prove that she knew what she was saying was false.
That's what I gathered from here:
http://definitions.uslegal.com/s/slander/