ASP Classic / JS multi select filter

Question

I am trying to create a filter to display records assigned to multiple groups not just one as currently set up below. Maybe via a checkbox. Any assistance would be appreciated.

ASP classic with JS using microsoft access as database

<form method="get" name="frmJumpMenu" id="frmJumpMenu">
  <div align="right"><font face="Arial, Helvetica, sans-serif">
   <select name="jumpFilter" onChange="MM_jumpMenu('parent',this,0)">
    <option value="">** Filter Group **</option>
    <option  value="set_group.asp?filter=%%">Show All</option>
    <% while (!rsGroups.EOF) { %>
     <option value="set_group.asp<%=(rsGroups.Fields.Item("GROUP").Value)%>"><%=(rsGroups.Fields.Item("GROUP_LABEL").Value)%></option>
    <% rsGroups.MoveNext(); } 
      if (rsGroups.CursorType > 0) { 
        if (!rsGroups.BOF) rsGroups.MoveFirst();
      } 
      else { rsGroups.Requery(); 
      } %> </select> </font>
  </div> 
</form>         

    <% var rsUnack__filter = "%%"; if (String(Session("usergroup")) != "undefined" && String(Session("usergroup")) != "") { rsUnack__filter = String(Session("usergroup")); } %>

    function MM_jumpMenu(targ,selObj,restore){ eval(targ+".location='"+selObj.options[selObj.selectedIndex].value+"'"); if (restore) selObj.selectedIndex;}

The code on set_group.asp

<% Session("usergroup") = Request.QueryString("filter").item %>

Setting up the filter to filter client side isnt an option as the page has a refresh every 60 seconds due to records being added / amended regularly. I am trying to learn so any help or advise would be appreciated. I know the select tag will need the multiple attribute.