Group the trigger to the group leader ... yes, as if the trigger was a unit. Now you get new options in the On Activation box. Select what you like, sounds like whole group not present in this case. In the On activation box write
[] exec "myscript.sqs"
myscript being a script that does whatever you want to happen. It could call in an arty strike on the group's position, or just setdammage 1 all the members, or whatever. There are plenty of arty scripts in the Ed Depot.