You can do the onPlayerLeave using default events:
0 onPlayerTouch > cancelEvents
0 onPlayerTouch > setColor > red
400 onPlayerTouch > setColor > white
That'll make a brick red while you're on it, and white when you're not.
You could try doing some sort of print counter and when it overflows, it collapses the bridge. Maybe increase by 1 ontouch, and decrease 200ms after. Then you can adjust how high the counter goes to make sure there are X many people on it.