In Calendar, click Home.
Mac users click the File menu > Open > Shared Calendar... (then skip to step 3)
In the Manage Calendars group, click Add Calendar, and then click Open Shared Calendar.
If you don't see Add Calendar, at the right end of the ribbon, select Add > Open Shared Calendar.
Type a name in the Name box, or click Name to select a name from the address book.
The shared calendar appears next to any calendar that is already in the view.
After you access a shared Calendar for the first time, the Calendar is added to the folder pane. The next time that you want to view the shared Calendar, you can click it in the Folder Pane.
If the other person whose Calendar you want to open hasn’t granted you permission to view it, Outlook prompts you to ask the person for permission. If you click Yes, a sharing request email message opens automatically. The message requests the person to share his or her Calendar with you and also provides the option to share your default Calendar with him or her.
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.
Microsoft 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.