Yup, this seems to work fine:
_trg = createTrigger ["EmptyDetector", getPos player];
_trg setTriggerArea [50, 50, 0, true];
_trg triggerAttachVehicle [myUnit];
_trg setTriggerActivation ["GROUP", "PRESENT", true];
_trg setTriggerStatements ["this", "hint 'YUP'", "hint 'NOPE'"];
I set the group to move into the trigger, got the Activation hint, killed the group leader ("myUnit"), got the Deactivation hint (one group member was not alive), followed by the Activation hint again (dead member removed, so all group members present and alive again).