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));
}