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This script will prompt for the user to granted permissions, then grant them permissions to ALL mailboxes.

$name = Read-Host 'What user are we granting permissions to? Input full email address.'
foreach($user in (Get-Mailbox -RecipientTypeDetails UserMailbox)) {

$cal = $user.alias+":\Calendar"

Add-MailboxFolderPermission -Identity $cal -User $name -AccessRights Author

}