First you need to connect to Office365.

Next, simply run this commandlet:
Add-MailboxFolderPermission calendar@company.com:\Calendar –User guy@company.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 calendar@company.com:\Calendar –User guy@company.com

Finally, to remove permissions:

Remove–MailboxFolderPermission –Identity calendar@company:\calendar –user dude@company.com