I decided to instead use CS_EnumUnitsInAOE to get a group of units, then cycle through them with:

JASS:
local group lNearUnits
local unit lUnit = null
loop
set lUnit = FirstOfGroup(lNearUnits, lUnit)
exitwhen lUnit == null
call GroupRemoveUnit(lUnit)
endloop
I was stuck on how to execute code on each of the units in a group. This lets me do so, while circumventing the restrictions of ForGroup.