If you're using an encounter trigger, you can select the encounter on the map (or in the menu bar on the left), and then right-click on the ground where you want the encounter to spawn. It will create a Spawn Point token, kind of like a Waypoint but it looks distinct, which you can grab and drag to wherever you want the creatures to spawn in.
After looking at the lexicon, I dont see a way to spawn a creature from an Encounter resref. I was hoping something like CreateObject(OBJECT_TYPE_ENCOUNTER, "zombiegrp001", lTarget), or something would exist similar to CreateTrapAtLocation(), but I see nothing.
Comments
You can spawn a mob that way - the engine ensures that each spawn is at a legal location next to the PC but without clipping.