For all users a reading permission is assigned on the free/busy times in the personal calendar when configuring the Exchange inbox. Thus, every Exchange user can access the information in any calendar. Any further descriptions like subject or location remains hidden. To change those standard access rights, you need to open the MS Outlook Program since it is not possible to change the settings in the Outlook Web App (OWA).
Within the MS Outlook calendar select your personal calendar by clicking on it with the right button of the mouse. Choose "Properties" in the now appearing pop-up menu.
Within the register "Permissions" change the reading rights for the "standard calendar" to "None". Confirm your changes by selection "OK".
To still use the option for calendar management you need to give reading authorizations to the members of the work group. Select "Add" and chose the members from the address book.