$Domain = 'domain'
$UserName = 'Username'
$Servers = 'server1','server2','server3','server4'
foreach($Server in $Servers) {

     if(Test-Connection -computername  $Server -Count 1 -Quiet) {

         $SeverGroup  = [ADSI]"WinNT://$Server/Administrators"
         $SeverGroup.Add("WinNT://$Domain/$UserName")

         # List all the memebers of Admin gorup
         $Group.psbase.Invoke("Members") | foreach {$_.GetType().InvokeMember("Name", 'GetProperty', $null, $_, $null)}
     }
}