Concatenating array into cells based on conditions

Am looking for help creating the output below in Excel 2007. I have a large number of data points; the example is a subset for discussion purposes.

For each row in the table on the left, output an "x" (or other character) to the corresponding cell in a new array (shown on the right), concatenating and separating by commas, if there is more than one row with the same combination of values.

In the example, there are three rows (3,5 and 6) with an Imp value of 4 and a Rat value of 3. I would like the display to show three "x's" (or another character) separated by commas in the cell that corresponds to Imp of 4 and Rat of 3.

Attachment 483

I'm an experienced Excel user, but not in macros/VBA, so may need some guidance there as well. Thank you.