If you are using a group selection formula to accomplish this, your formula should look like this:
if {?zeroBalance} = "Yes" then
Sum({@curAmount}, {@groupopt3})<> 0 else
(Sum({@curAmount}, {@groupopt3}) =
sum({@curAmount}, {@groupopt3})
If the user says "Yes" (eliminate groups with zero balance) then this will display only those groups with balances <> 0. If {?zeroBalance} <> "yes" then no group select will be made, i.e., all groups will display.
-LB