NPCs will have a chance to spawn within the spawn radius around the player every hour.
NPC groups are spawned in batches. Either raider group or survivor group will spawn.
The NPCs in the group will be added even if the NPC limit is reached.

NPCs that are not in the players group will be deleted when the grid square that they are standing on is unloaded by the player moving out of render distance.
NPCs that are in the players group will be unloaded and reloaded when the grid square is reloaded. Unloaded NPCs are basically like unloaded zombies: they are just mapped there doing nothing. No hunger, thirst, activity, whatsoever.