SC2Mapster

Any Unit enter Region
Set a Boolean Value to True/False based on the Owner of (Triggering Unit)
Check whether all Players Variable is set to True
If so do whatever Action

To determine if all active players have the unit it might be easiest to make

tempCounter = 0
COUNTER = YourNumberOfPlayers
Boolean[COUNTER]

Any Unit enter Region
Set Boolean[triggeringplayer] = True

Loop through Boolean
for(int i=0; i < COUNTER; i+1)
If Boolean[i] == true
Set tempCounter + 1
Else do Nothing

Outside loop
if(tempCounter == Number of Active Players)
{
Then all players have a unit in the region
}

Then Create the same thing but
Any Unit Leaves Region
Set Boolean value to false