Sharing has never been something I've used very much. I always make my sites public and I limit collaborator privileges (call me a control freak). Nevertheless, it is always great to be able to do something more easily. Google Sites sharing is now similar to that in other Google Apps like Google Docs.
Remember your options:
- Anyone can see your site (public) *will be indexed eventually and then not very well
- Only those with a link can see your site (semi-private) *will not be indexed
- Only those given permission can see your site (mostly-private) *will not be indexed
For Google's post on website sharing see http://googleappsupdates.blogspot.com/2010/11/simplified-sharing-experience-extended.html