Try something like this
$Credential=Get-Credential
Get-VM |Where-Object{$_.Guest.GuestFamily-like"win*"-and$_.PowerState-eq"PoweredOn" }|
ForEach-Object-Process {
Invoke-Command-ComputerName "$($_.Name).<domainname>"-Credential $Credential-ScriptBlock {
Get-LocalGroupMember-Group "Administrators"
}
}|Export-Csv-Path 'C:\Users\xxxxxx\Desktop\localAdmins.csv'-NoTypeInformation -UseCulture