Note: The easiest way to do this is to run Plone 3 and use the specialized Intranet workflow instead of following this how-to. If you're using Plone 2.x, the below is the easiest way:

These are the steps to create a Private Plone site

• from the Zope Management Interface for my Zope site I added a Plone Site
• From the Plone Site root I went to the security tab and made it so that Add portal member was only available to Manager and Owner with no Acquire. This removes the Join button.
• From the Plone Site root I went to portal_workflow
• Note that two workflows are actually used in Plone, folder_workflow and plone_workflow
• Go to the Contents tab
• Click on the folder_workflow
• Click on States tab
• For each state listed:
  • Click on the state name
  • Click on the Permissions tab
  • Change any Anonymous checked areas to Authenticated and no Acquire
  • Save the changes
  • Repeat this for all the states
• Also do the above for the next workflow plone_workflow
• Back at the portal_workflow press the Update Security Settings button to make sure the changes propagate through any existing items.

And that should be it!

People who have not logged in see the pleasant Plone "Please Log In" page for Home and Members. "No News Posted" under News. And searches come up with "nothing found".

Log in with a valid account and the news, home page, member list are all visible and the search returns items.