Follow these steps to share your calendar:
1. Login to Office 365 OWA
2. Click on the Calendar icon.
3. Click Share icon
4. Select which calendar you would like to share.
5. In the Share with: field, type the email address(s) of the users that you wish to share your calendar.
6. Choose what permissions you wish each user to have using the drop down menu next to their name.
7. Click Send
This process can even be automated through a tool as well. You can download this Office 365 Exporter Tool and then easily can export Office 365 mailbox content to Outlook PST file format. This tool is fast and easy to perform.