First you need to connect to Office365.
Next, simply run this commandlet:
Add-MailboxFolderPermission email@example.com:\Calendar –User firstname.lastname@example.org –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 email@example.com:\Calendar –User firstname.lastname@example.org
Finally, to remove permissions:
Remove–MailboxFolderPermission –Identity calendar@company:\calendar –user email@example.com