Outlook Calendar Access
OFFICE 365 OUTLOOK CALENDAR ACCESS
To access a user's calendar, User that has access needs to add user who needs access as a delegate in his Outlook and assign with the access permission to calendar only.
Select the File tab, then select Account Settings > Delegate Access.
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In the dialog, select Add.
If Add doesn’t appear, an active connection might not exist between Outlook and Exchange. The Outlook status bar displays the connection status.
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Type the name of the person whom you want to designate as your delegate, or search for and then click the name in the search results list.
Note: The delegate must be a person in your organization's Exchange Global Address List (GAL).
More Resources:
https://support.microsoft.com/en-us/office/allow-someone-else-to-manage-your-mail-and-calendar-41c40c04-3bd1-4d22-963a-28eafec25926#__toc305134742