Hi @IMK
You may use the following script to get all user mailboxes and assign Reviewer permission of the calendar folder to the security group:
$users = Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox
Foreach($user in $users)
{
Add-MailboxFolderPermission -Identity $User":\Calendar" -User <SecurityGroup> -AccessRights Reviewer
}
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