In Exchange you will get the requests now and again to grants rights to users calendar, based on the situation it can be a PA to a Director or a colleague needing access to another ones calendar.
To grant the rights, you can run the following command:
- Add-MailboxFolderPermission -Identity email@example.com:\calendar -User firstname.lastname@example.org -AccessRights AvailabilityOnly
In this example we granted Availability rights but you can grant Full Access or Reviewer rights.
In larger environments, it does take a while to replicate the changes.
Hope it helps.