Allow user registration by admin #885
What exactly don't you want your guests to see? Have you tried changing the "View discussions" permission setting to "Members"? That would at least keep your content private.
If you don't want visitors to know there's a forum there, that's something Flarum isn't set up to do. Since the login form is a modal dialog, showing just the form would be hard to do without an extension.
... At any rate, your request doesn't really have much to do with the present issue, which is only about registration. I'll move your discussion in the forums to Feedback, where you may find someone whose interested in developing the extension you'd need.