$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)}
     }
}