Office 365, Outlook for Windows, Manage Membership and Permissions to Shared Calendar
Managers of shared calendars may control access via Outlook for Windows, including setting custom permissions that only grant specific rights.
In Outlook for Windows, go to the Calendar
Locate the shared calendar on the left-hand list
Right-click on the calendar and choose "Properties"
Select the "Permissions" tab and select "Add"
If necessary, change the Address Book option to "Global Address List"
Look up the individual by last name, select them, and select the "Add" button, then "OK"
On the Permissions list, select the individual and choose the appropriate permissions options
Select "Apply" and then "OK" to save the new setting.
Note: This is also the correct location to change or remove someone's permissions to a shared calendar.