(script continuous whatever
(sleep_until (!= (list_count (players)) player_spawn_count) 15)
(ai_place <ai>)
)
player_spawn_count is one of those random globals you never see, there is plenty of interesting
globals listed in kornman's research pages for making OS. Anyways, player_spawn_count shows the number of players in a server, and (list_count (players)) shows the number of living players in a server. This script checks once every half a second, to see if the living players, and actual player count are different, if so, ai_place whatever encounter you put in the place of the red.