$ProjectX = Get-DynamicDistributionGroup -Identity "TDG1"
$Groups1 = Get-Recipient -RecipientPreviewFilter ($ProjectX.RecipientFilter) -OrganizationalUnit ($ProjectX.RecipientContainer)

$ProjectX = Get-DynamicDistributionGroup -Identity "TDG2"
$Groups2 = Get-Recipient -RecipientPreviewFilter ($ProjectX.RecipientFilter) -OrganizationalUnit ($ProjectX.RecipientContainer)


Foreach ($group1 in $groups1){
    Foreach($group2 in $groups2){
        $temp = $group1.Name + ":\Calendar"
        Add-MailboxFolderPermission -Identity $temp -User $group2.Name -AccessRights AvailabilityOnly
    }
}