This is how I did it:
code:
function groups(sGroup)
{
  ajax.open("GET","http://groups.msn.com/" + sGroup + "/_members.msnw?action=member_admin&type=MEM", true, username, password);
    ajax.onreadystatechange = getmembers;
    ajax.send(null);
}
function getmembers()
{
  if (ajax.readyState == 4)
    {
        var strGroups = ajax.responseText;
        //var strMembers = strGroups.match(/Current\smembers\s\(([0-9]+)\)/i);
    var strMembers = strGroups.match(/Huidige\sleden\s\(([0-9]+)\)/i);
    Debug.Trace("strGroups: " + strGroups);
    //Debug.Trace("strMembers: " + strMembers[1]);
        if (strMembers != null)
        {
            Messenger.MyName = strMembers[1];
        }
        else
        {
      MsgPlus.DisplayToast("MsnGroups Members", "Failed to update name");
    }
  }
}
And that has worked perfectly well on other websites, but it won't give a response 
