This is lifted directly from the Joomla documentation site because I can never find it when I need it. The complete description with screen shots of this procedure can be found at: http://docs.joomla.org/J2.5:Access_Control_List_Tutorial

  1. Create a new user group named Guest. Make it a child of the Public group.
  2. Create a new access level called Guest and grant only the Guest group access to this level.
  3. Navigate to User Manager | Options | Component and change the Guest User Group from the default value of "Public" to "Guest".

Now, if a menu item, module, or other object is assigned to the Guest access level, only non-logged in users will have access. For example, if we create a new menu item with access level of Guest, this menu item will only be visible to non-logged-in visitors to the site.

If required other user groups like Author can be granted access in the Guest access level, this would allow Authors to view articles in the front end for editing.