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In the Outlook Online Web App go to the calendar
Click on the gear (settings) in the upper right corner
Under View select Shared calendars
In the pull down Select a calendar, select the calendar you want to share
Click the red Share button
In the provided space enter an email address or contact name you want to share the calendar with.
In the pull down window below you can select the permission level you want give them.
Click the Share button.