You need a switch trigger. Basically, make your trigger type switch: now synchro it with the appropriate waypoint. The group will not pass that waypoint till the trigger is fired. However, when the trigger is fired they will go on to the waypoint after it even if there are a couple of intervening waypoints.
Search the forum on switch triggers, they come up all the time. The good tutes will help you too.
Highlander's way will work too of course. You don't actually need the script in his example - you could simply write go=true in the On Activation field of the trigger, with "go" in the condition field of a waypoint.