To get Item details or  Product based on Item group in Ax 2012 you can try below code in job. In this job Test_InventItemGroup is query on which you need to include inventItemgroupItem and inventTable. I have pass the range TT as item group same way you can use in your customization.
   Query  query;  
   QueryRun qr;  
   QueryBuildDataSource qbds;  
   QueryBuildRange queryBuildRange;  
   InventItemGroupItem IntventTest,InventTest1;  
   ;  
   query = new Query(queryStr(Test_InventItemGroup));  
   qbds = query.dataSourceTable(tableNum(InventItemGroupItem));  
   queryBuildRange = qbds.addRange(fieldNum(InventItemGroupItem,ItemGroupId));  
   queryBuildRange.value("TT");  
   qr = new QueryRun(query);  
   while(qr.next())  
   {  
     IntventTest= qr.get(tableNum(InventItemGroupItem));  
     select ItemId from IntventTest1  
       where IntventTest1.ItemGroupId == IntventTest.ItemGroupId;  
     info(strFmt("%1",IntventTest.ItemId));  
   }