As a private installation, users cannot sign up for an account. I've hidden the option in a few places (by removing links to Contact administrators, etc.), but if my understanding is correct, if the installation mode is marked as "Private" users should not see options to sign up. I see it in various login screens, or if users attempt to visit a URL without having logged in first. It is confusing for users to see this phrasing, and I think it would be an improvement to remove mention of signing up or links for same if running in private mode.