Below is the code I ended up using with some help from Tech Support.
This goes in the code block:
total = 0
LocationID = 0
def accumulate(locID, Inc):
global total, LocationID
if locID != LocationID:
total = 0
LocationID = locID
if total: total = total*Inc
else: total = Inc
return total
In the expression I use accumulate(!LocationID!,!IncField!)
The LocationID is my grouping Field and the IncField is the one that gets multiplied together.