We used CalDAV Synchronizer, which is free. Download and install the CalDav Synchronization plugin
Two-way synchronization is only available for people using the Microsoft Outlook desktop application on Windows, with the CalDAV Synchronization plugin installed. Subscribe with two-way synchronization (CalDAV) This option requires a free CalDAV Synchronization plugin for Outlook, and is only available for Windows users. There are two ways to synchronize your calendar in Outlook: Subscribe to a Team Calendar from Outlook (Windows)