Use a counter. Connect the zone to the counter to increment it when a player walks into the zone. Have the zone transmit on a channel that decrements the counter when a player walks out of the zone.
Create a system to count the amount of players in game if you haven’t already. Make both counters in the counting system and the zone system update a property. Connect the zone to a trigger that checks if the player counter’s property is equal to the zone counter’s property. If so, broadcast message on channel.
What do you mean about the 5 second time thing?