part of array formula based on data change in another column?

I have the following two worksheets and am trying to retrieve the ItemID in the "New" worksheet from the "Item-Group" worksheet.

The first few items in column C on the "New" worksheet tab are correct, but I want to fill in the rest of the table (which is far longer in practice).

Column C's formula is

=INDEX('Item-Group'!$A$2:$F$126,SMALL(IF(Groups=B2,ROW(Groups)-1),ROW(A1)),1)

, entered as an array formula, and then copied down the column. The problem is, for C4, the formula has ROW(A3), but since that's where group b starts in the column, I want it to go back and restart at ROW(A1). Is there a nice way to do this?