Noooo - leave the _x as is; look up the ForEach command in the command reference.
The trigger as I've written it doesn't need the heli name or the group name. Basically, when the player is not in a vehicle (in Flashpoint terms a man is a vehicle, too, so we're looking for when HE is in himself, not the chopper) each unit in the player's group is given the UnassignVehicle command, then the DoFollow. The '_x' is the variable that takes the value of each group member.
To use the height, try a condition of:
((GetPos heliname) Select 2) > 3
or, to build on what we had before:
((GetPos Vehicle player) Select 2) > 3
Because 'Vehicle player' and 'heliname' refer to the same unit.
GetPos gives an [x,y,z] position coordinate. The third element of the array: z, counting from 0, is extracted by array Select 2 - this is the height above ground level. Again, it will be screwed up if the helicopter is on the ground at the beginning of the mission.
Experiment and use the command reference - they're the tricks.