How to add a item group using scripting

I have been trying to add an item group item that has over 10 member items to an invoice using server side suitescript, but I cannot get it to work, here is the code I am using

newinvoiceRec.setLineItemValue('item','item', 1, 3460); // item group item
newinvoiceRec.setLineItemValue('item','custcol_type',1,24); // transaction column - Product
newinvoiceRec.setLineItemValue('item','custcol1',1,1); // transaction column - Users
newinvoiceRec.setLineItemValue('item','quantity',1,1); // transaction column - quantity
newinvoiceRec.setLineItemValue('item','amount',1,1); // transaction column - amount

id = nlapiSubmitRecord(newinvoiceRec,true);

NetSuite throws the following error
You must enter at least one line item for this transaction

the same code above works fine for a non 'item group' item. Can someone please show me the correct code to use. Thanks in advance for any help extended.