1)  Find the calendar, you want to change access permissions for, in the left list of calendars and hover over the right side until you see the 3 vertical dots, and click the dots:

2)  Then click “Settings and sharing”:

3)  Then under “Access permissions”, beside the “Make available for Manitoulin Group of Companies”, it is likely that “See all event details” is set.  Click there, then choose “See only free/busy”

There is no need to save as calendar setting save as soon as changes are made.