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?

Try

=IFERROR(INDEX('Item-Group'!$A$2:$A$25,SMALL(IF('Item-Group'!$C$2:$C$25=New!A2,ROW($A$2:$A$25)-ROW($A$2)+1,""),ROWS(COUNTIF($A$2:A2,A2)))),"")

Enter with Ctrl+Shift+Emter

That gives the correct ItemID for the first instance where the group name changes, but the in between ones are just the same as the first one.

That may not be clear.

What I want is this:
a-2-43
a-2-45
b-6-841
b-6-842
b-6-894
b-6-895
b-6-896
b-6-897
c-8-843

but your formula is showing this:
a-2-43
a-2-43
b-6-841
b-6-841
b-6-841
b-6-841
b-6-841
b-6-841
c-8-843

Oh, I think I found it - you don't want ROWS in the last part, just the COUNTIF statement.

Thanks!