http://support.google.com/sites/bin/answer.py?hl=en&answer=90594 (Click on the links on the left for details about different topics).
https://www.google.com/a/cpanel/..YOUR DOMAIN NAME../ServiceSettings#ServiceSettings/service=sites&subtab=0
to modify sharing restrictions. (KCcloud solutions has provided a useful links generator for other useful pages)
See this page for details of how to use page level permissions
Very occasionally, it is necessary to use the 'old' style sharing interface when the current interface is not working properly.you can get back to the old sharing interface: add /system/app/pages/admin/sharing to the end of your site's URL and it will take you to the old sharing interface.
https://sites.google.com/a/pjrprojects.co.uk/sitesguide/system/app/pages/admin/sharing for Google Apps Sites or
https://sites.google.com/site/..SITENAME../system/app/pages/admin/sharing for non Google Apps Sites
If you need a mechanism to generate a list of Site members, then see this script
Some Google Apps users have experienced problems when trying to share their Sites. Apparently it is necessary to either use the 'old' interface (see above) or have the Google Docs service enabled in your Google Apps domain. So check whether Google Docs is enabled and try switching it on if it isn't already.