I'm looking into setting up a forum for a writers group I attend; this would be for sharing stories amongst ourselves. In order to retain first rights, this has to be within a hidden forum, where the wider internet can't gain access to it.
Assuming that the server it's hosted on is trustworthy, are these forums actually secure for these purposes? I'm considering using Simple Machines forum software, but this is a more general question: how sure can I be that the information on these forums will remain hidden? There is obviously the risk that a member will spread stuff around (unlikely, as we all know each other in the real world), but can the software generally be considered secure?
Will any hidden forums be cached by, for example, Google? If not, why not - is it a genuinely secure feature of this kind of software, or the equivalent of asking nicely (like robots.txt)?
I should perhaps clarify that I am going to be the sole administrator of this website, and the host is someone I believe to be trustworthy, so problems with terms of service etc. are irrelevant to me. I am asking if the software can be considered secure.