i wanted this very same thing a map i was making and it took me ages to figure it out.
this is how i did it
i layed a east man on the map in the middle of the sea.
and in his init line i put ...... east1=gfroup this;.....
then i made a sqs example below
;;;;;;;;;;;;east1.sqs;;;;;;;;;;;;;;;;;;;
_units1 = units east1
"_x setunitpos up" foreach _units1
"_x allowfleeing 0" foreach _units1
#loop
? NOT(alive player): exit
?(("Alive _x" count _units1) == 0): goto "create"
~random(5)
goto "loop"
#create
"OfficerE" createunit [getpos a3, east1, "e1 = this", 1, "LIEUTENANT"]
"SoldierEG" createunit [getpos a3, east1, "e2 = this", 1, "SERGEANT"]
"SoldierEB" createunit [getpos a3, east1, "e3 = this", 1, "CORPORAL"]
"SoldierEB" createunit [getpos a3, east1, "e4 = this", 1, "CORPORAL"]
"SoldierEB" createunit [getpos a3, east1, "e5 = this", 1, "PRIVATE"]
"SoldierEMG" createunit [getpos a3, east1, "e6 = this", 1, "PRIVATE"]
hope that helps you, it helped me.