By default, Google Apps/ Google Workspace user calendars are not shared externally. Fortunately, it’s a simple fix if you know where to look. Here are the steps you can take to enable it in the Google Apps admin console.
Log in to the Admin Console.
Click Apps > Google Apps > Calendar
Under Share settings, change the default to one of the following options:
Share all information, and outsiders can change calendars (my new default)
Share all information, and allow managing of calendars
Unless you plan to delegate management of the Calendar, there really is no need to select the fourth option. Be sure to click Save Changes once you make the changes.
Once the calendar sharing settings have been completed, the Private Address links for iCAL and XML should appear for your end users.