This might be a somewhat impossible situation to resolve - in that if you explicitly set the permissions on the site to prevent access to the public then reaching the register page is impossible. There maybe some sort of workaround i could do for this, but...

You might find things work better if you use the Members Area feature to have a section of the site that is restricted to members, but keep the public pages of the site to allow you to invite/register users, and have an introduction page etc. Then when users log in they get access to the Members Area.

This is the more "standard" way of doing things, which quite a few sites use.

Thanks

Joe