I don’t see why you couldn’t use a defederated version of Lemmy as a kind of private forum. That said, it’s most likely easier to use a good old phpBB forum, there is still tons of companies offering you to host one, it’s a bit outdated and not that popular anymore, but various clubs still use one because it’s what we set 15 years ago, and it’s still working (and to be fair, I like that forum keep information over time while it’s a hell to search old thread in whatsapp/discord). A lemmy might be a more modern form of a forum for a club/school/whatever. I don’t know whether you have “private” forum like you would with the good old phpBB like forum