I guess you do not want the reinf to simply teleport, so they can be somewhere and waiting for action...
If it is ok, crate the other group as well in the editor, place them where you want, hiding somewhere. Give them waypoints leading them to where they will attack. Name your first enemy group to DrugDealerGroup01.
Create a trigger and synchronize to the reinf's first waypoint.
Set the trigger:
size : 0
activation: none
type: none
Condition: count (units DrugDealerGroup01) < 1
It means when every units of DrugDealerGroup01 will killed, the other group will move to it's next waypoint.