Hi Gandha,

I've just started using the system too, so my apologies if I've misunderstood your question or my response is misleading, but if I understand you correctly, take a look under the Website Admin menu tab "Users and Permissions". Here, as you probably know, you can view and manage all your members and groups etc, you'll also see an option for "Permissions and Sharing". Under this option you can grant the various Read, Contribute or Admin privileges to your various groups and importantly, one of these groups is called "The Public" which I would guess on your site is checked with at least the privilege to read? If all the privileges to this group are revoked (unchecked), the site will only be accessible to registered members. Also I noted that one's listing on the Voice Directory of sites. looses it's hyperlink too! Hope this helps,

Here's a link to the detail in the manual 

Cheers PC.