Consider the fact that, if you were to make your Calibre library publicly available on the Internet, then unless all your books were in the public domain you would in effect be setting up your own pirate book site. You would need to provide an authentication mechanism to ensure that only you could access it, otherwise you could find yourself in very hot water, legally. That's the reason that the Calibre server is only visible to your own private network. It's also worth bearing in mind that "sharing your library with a couple of other people" may well constitute copyright infringement, unless the publisher permits such sharing.
Last edited by HarryT; 12-30-2012 at 04:36 AM.