Get the members of the Local Administrators group

Get-LocalAdmins.ps1

Function Get-LocalGroupMembers
{
param(
[Parameter(ValuefromPipeline=$true)][array]$server = $env:computername,
$GroupName = $null
)
PROCESS {
    $finalresult = @()
    $computer = [ADSI]"WinNT://$server"

    if (!($groupName))
    {
    $Groups = $computer.psbase.Children | Where {$_.psbase.schemaClassName -eq "group"} | select -expand name
    }
    else
    {
    $groups = $groupName
    }
    $CurrentDomain = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain().GetDirectoryEntry() | select name,objectsid
    $domain = $currentdomain.name
    $SID=$CurrentDomain.objectsid
    $DomainSID = (New-Object System.Security.Principal.SecurityIdentifier($sid[0], 0)).value


    foreach ($group in $groups)
    {
    $gmembers = $null
    $LocalGroup = [ADSI]("WinNT://$server/$group,group")


    $GMembers = $LocalGroup.psbase.invoke("Members")
    $GMemberProps = @{Server="$server";"Local Group"=$group;Name="";Type="";ADSPath="";Domain="";SID=""}
    $MemberResult = @()


        if ($gmembers)
        {
        foreach ($gmember in $gmembers)
            {
            $membertable = new-object psobject -Property $GMemberProps
            $name = $gmember.GetType().InvokeMember("Name",'GetProperty', $null, $gmember, $null)
            $sid = $gmember.GetType().InvokeMember("objectsid",'GetProperty', $null, $gmember, $null)
            $UserSid = New-Object System.Security.Principal.SecurityIdentifier($sid, 0)
            $class = $gmember.GetType().InvokeMember("Class",'GetProperty', $null, $gmember, $null)
            $ads = $gmember.GetType().InvokeMember("adspath",'GetProperty', $null, $gmember, $null)
            $MemberTable.name= "$name"
            $MemberTable.type= "$class"
            $MemberTable.adspath="$ads"
            $membertable.sid=$usersid.value


            if ($userSID -like "$domainsid*")
                {
                $MemberTable.domain = "$domain"
                }

            $MemberResult += $MemberTable
            }

         }
         $finalresult += $MemberResult 
    }
    $finalresult | select server,"local group",name,type,domain,sid
    }
}

#"server1", "server2" | Get-LocalGroupMembers -group administrators | ft -autosize
Get-LocalGroupMembers -group administrators | ft -autosize

Reference: http://serverfault.com/questions/648506/get-all-local-groups-members-and-account-status-locked-and-disabled