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Even if the other person has created an additional calendar, you can open only the person's default Calendar.
To remove a calendar from the Other Calendars list, right-click the calendar, and then click Delete Calendar.
The owner of the calendar items controls who can see the items and change them.
Office 365 cross-premises shared calendars are read-only, so clicking New Meeting with All will not detect and add owners of those shared calendars. Instead, manually add them to the invitation.
Please can you open the calendar view in outlook
Then from the options on the ribbon at the top choose Add Calendar \ from address book
Type new in the search and click New business and corporate so it appears at the bottom and then click ok.
The calendar should then be visible on your shared calendars list.
Many thanks.