Overview
You can grant access to your Calendar or to a Calendar of a Mailbox that you have been granted Delegate Permissions to.
Instructions
Outlook Web Client
Login to your Outlook web email account at www.office.com
Open the Calendar Application from the Waffle Menu in the top right hand corner of your screen. You may have to Click All apps if you've never used Calendar before
Once the Calendar program is open, Click your Emoji Circle in the top right hand corner and click Open another mailbox
Type the name of the Mailbox you want to get access to
Example
Click OPEN
In the top menu, click Share
Enter the email address of the user you want to share this calendar with, and choose the permissions.
Click Share