This script will query the computers in (Get-ADComputer -Filter *) and then output the members of BUILTIN\Administrators to c:\psresults\ListOfLocalAdministratorsGroup.txt

Get-LocalAdministrators.ps1

#The Third section will query each computer in the ListOfComputers.txt to get the members of the local group Administrators
#$Servers = (Get-ADComputer -Filter *).name
$output = 'c:\psresults\ListOfLocalAdministratorsGroup.txt'
$results = New-Object System.Collections.ArrayList

$objSID = New-Object System.Security.Principal.SecurityIdentifier("S-1-5-32-544")
$objgroup = $objSID.Translate([System.Security.Principal.NTAccount])
$objgroupname = ($objgroup.Value).Split("\")[1]

foreach($server in (Get-ADComputer -Filter *).name)
{
    $admins = New-Object System.Collections.ArrayList
    $group =[ADSI]"WinNT://$server/$objgroupname" 
    $members = @($group.psbase.Invoke("Members"))
    $members | foreach {
        $obj = new-object psobject -Property @{
            Server = $Server
            Admin = $_.GetType().InvokeMember("Name", 'GetProperty', $null, $_, $null)
        }
        #$obj
        $admins.Add($obj)
    } 
    $results.Add($admins)
}
$results | Out-File $Output