Set/Share user calendar access

First connect to the Office 365 using instruction in the below url

http://yoursolutionshut.blogspot.in/2015/05/managing-office-365-using-powershell.html

Then run this command

Add-MailboxFolderPermission -Identity arun@abc.com:\Calendar  -AccessRight ReadItems  -User kumar@abc.com

Above command will set the read accss to Arun's calendar to Kumar.
Now Kumar can add and read Arun's calender

How to revoke the access

Remove-MailboxFolderPermission -Identity arun@abc.com:\Calendar  -User kumar@evolving.com

Above command will revoke the changes.