This might be what you're looking to do:
$BanMeDate = (Get-Date).Date.AddMonths(-4)
Get-ADUser -Filter * -Properties PwdLastSet |
ForEach-Object{
if ([DateTime]::FromFileTime($_.PwdLastSet) -le $BanMeDate){
Add-ADGroupMember -Identity BannedUsers -Members $_.SamAccountName -WHATIF
}
}
Be sure you TEST THIS! When you're satisfied that it isn't going to destroy your organization, remove the "-WHATIF" parameter.