This is a tutorial on how to add calendar permissions in Office 365 for your users via Powershell. You can add permissions onto a specific mailbox, or you can add it onto a security group.
By default, Exchange (and Office 365) users can’t view messages or calendar items of other users. The only permission that is provided to all users by default is the ability to view free/busy information in the calendar of other users (AvailabilityOnly role).
The first is step is to launch Windows PowerShell. We recommend running it as Administrator.