First you need to connect to Office365.
Next, simply run this commandlet:
Add-MailboxFolderPermission firstname.lastname@example.org:\Calendar –User email@example.com –AccessRights Author
Your accessrights options are: Owner, PublishingEditor, Editor, PublishingAuthor, Author, NonEditingAuthor, Reviewer, Contributor, AvailabilityOnly, LimitedDetails
If you’d like to get permissions, you can simply use:
Get-MailboxFolderPermission –Identity firstname.lastname@example.org:\Calendar –User email@example.com
Finally, to remove permissions:
Remove–MailboxFolderPermission –Identity calendar@company:\calendar –user firstname.lastname@example.org