Place several sent_spawner entities in the map where you want NPCs to spawn, set the key value "npc" to a string containing NPCs you want to spawn near it, e.g
"npc_combine_s npc_hunter npc_fastzombie". If you don't set it, it will default to whatever is set in the gamemode config.
There should be something stopping players from camping the NPC spawn. Radiation, teleporter, etc.
Map needs to be properly noded for the NPCs to work.
Place something to stop players from camping the player spawn, don't kill them though. Something that pushes them off when they spawn is a good idea.
Don't forget to include plenty of building room, don't put any props in the map.