Legal Question in Constitutional Law in Ohio

Facebook discussions are not legally privileged. Calling them "secret" does not make it a crime for someone to reveal what was said. It also does not make evidence of what was said inadmissible in court.

You say that the information "was gathered without the consent of any member in the secret group", which seems implausible. How do you think the former members got the information? Do you believe they hacked their way in? You might be able to persuade a court to exclude evidence that was gathered dishonestly -- not because it's inadmissible, but because the court might decide allowing the use of such evidence would be inequitable. Don't count on that strategy working.

Ordinarily, speech can only qualify as harassment if it is directed at the alleged harrassee. That doesn't mean she has to be one of the initial recipients; it's enough if the speaker reasonably foresees that she will find out about it. Comments made in private which the speaker has no reason to think the subject will find out about shouldn't qualify. But they could be defamatory, which would be a distinct basis for a lawsuit.

Your statement that "a complaint had been filed" is pretty vague. What sort of complaint was it? Who was it filed with? Are you saying you've been sued? Or is it possible that these people just complained to the booster club, or perhaps to the school board or Facebook? Rules of evidence wouldn't apply in those settings, and you could not be held civilly or criminally liable by any of those bodies.

Since you refer to a meeting with the board of the booster club instead of a court date, it sounds like this is an internal matter for the club. If that's all it is, your concern about liability and admissibility seem misplaced.