var checked_boxes = "";
for(var i=0; i<total_checkboxes; i++) {
   var cur_box = document.getElementById('userbox_' + i);
   if(cur_box.checked) {
      checked_boxes += i + '|||';
   }
}
   var all_checkboxes = document.getElementById('all_checkboxes');
   all_checkboxes.value  = checked_boxes;