$DDGroup = Get-DynamicDistributionGroup "DST-Dept1-Staff"
$MList = Get-Recipient -RecipientPreviewFilter `
 $DDGroup.RecipientFilter -OrganizationalUnit `
 $DDGroup.RecipientContainer `
 | Select Displayname,PrimarySmtpAddress | Sort Displayname
$MList
Write-Host "Members count:" $MList.Count