how can select bulk numbers for sending bulk sms.i want to send sms to numbers when selecting a group.how?

///Enter your details 
        string username="XXXXXXX";
        string password="XXXXXXX";
        
        string SenderID = "XXXXXX";
        string Message = "Test Message";
        string dest_MobileNo = "XXXXXXXXXX";

        WebClient client = new WebClient();
        string baseurl = "http://123.63.33.43/blank/sms/user/urlsmstemp.php?username=XXXXXX&pass=XXXXX&senderid=XXXXX&dest_mobileno=XXXXX&message=XXXXX&response=Y;
        Stream data = client.OpenRead(baseurl);
        StreamReader reader = new StreamReader(data);
        string ResponseID = reader.ReadToEnd();
        data.Close();
        reader.Close();