Add-MailboxFolderPermission -Identity [email protected]:\calendar -User [email protected] -AccessRights Owner

Set-MailboxFolderPermission

(change the permission rights on exiting user.)

more details from TechNet:

https://technet.microsoft.com/en-us/library/dd298062%28v=exchg.160%29.aspx?f=255&MSPPError=-2147217396

https://technet.microsoft.com/en-us/library/bb125153%28v=exchg.160%29.aspx?f=255&MSPPError=-2147217396

https://blogs.technet.microsoft.com/benw/2015/01/21/the-mystery-of-calendar-permissions-explained/