Use this:
<?php
$MRTdbmADI_details = new WA_MySQLi_RS("MRTdbmADI_details",$dbUpdate,1);
$MRTdbmADI_details->setQuery("SELECT dbmADI.*, GROUP_CONCAT(dbmSite.dbmSite SEPARATOR ', ') AS SelSites FROM dbmADI LEFT OUTER JOIN MRT07 ON dbmADI.dbmADIID = MRT07.MRT07dbmADIID LEFT OUTER JOIN dbmSite ON dbmSite.dbmSiteID = MRT07.MRT07dbmSiteID WHERE dbmADI.dbmADIID = ?");
$MRTdbmADI_details->bindParam("i", "".(isset($_GET['dbmADIID'])?$_GET['dbmADIID']:"") ."", "-1"); //WAQB_Param1
$MRTdbmADI_details->execute();
?>
You had extra spaces and dollar sign in your code preventing it from being recognized.