Write-Host "What is the UPN of user or group that should be added: " -NoNewline
$upn = Read-Host

Write-Host "What permissions should be applied: " -NoNewline
$calendarPermission = Read-Host

Write-Host "Granting $upn $calendarPermission permission to all mailboxes"

$mailboxes = Get-EXOMailbox

$totalMailboxes = $mailboxes.Count
$processCount = 0

Write-Progress -Activity "Applying calendar permissions" -Status "Starting" -PercentComplete 0

foreach ($mailbox in $mailboxes) {
    $calendarPath = "$($mailbox.UserPrincipalName):\Calendar"
    $processCount++
    $percentComplete = (($processCount / $totalMailboxes) * 100)
    Write-Progress -Activity "Applying calendar permissions" -Status "Processing $calendarPath" -PercentComplete $percentComplete
    Set-MailboxFolderPermission -Identity $calendarPath -User $upn -AccessRights $calendarPermission -SharingPermissionFlags None
}