Index/Match Formula to return multiple results

Hi, So I've created a formula that automatically returns the PO number to the corresponding offer it is referencing...and vice versa. The issue I am having is that occasionally there are several PO's which are for a single offer, though the formula I've created only returns the first PO number that it finds when I'd prefer it to report every PO number that is connected to the single offer. I understand there is something to do with an array formula and using Control-shift-enter when entering the formula but I can't figure out what to do to report multiple PO numbers.

I'll attach a sample file, The formula I am currently using is:

=IFERROR(INDEX(Pos!$A$2:$A$22,INDEX(MATCH(A2,Pos!$H$2:$H$22,0),0)),"")

Thanks for all your help
Mike Stafford