ThreadPool.QueueUserWorkItem(new WaitCallback(LoadUsers));

private void LoadUsers(Object stateInfo)

    var site = new SPSite(testHarnessSettings.Url);

     var web = site.OpenWeb();

      foreach(SPUser user in web.AllUsers)

      {

       if (usersListBox.InvokeRequired)

       {

             usersListBox.Invoke(new MethodInvoker(delegate { usersListBox.Items.Add((user.LoginName)); }));

       }