I'm glad it was useful :)
Coincidentally, I needed a form of wording for our own forums, it's sometimes easier to draft something for someone else than for oneself.
My previously offered text is abridged slightly, a postscript. See the highlight at <
http://www.diigo.com/04q5m>.
FWIW you might try removing, or setting aside, any front/page foreground forewarning of restrictions. Simply allow the user to encounter the invitation, the form is pleasingly clear and simple so (my guess) you will receive few questions concerning authority to post.
Regards
Graham