setPos does help.
Place the waypoints where you want them and the group on teh little island. They'll try to follow the waypoints but not to the point of drowning, so they should just stand around on the island.
Place a gamelogic where you want them to materialise.
The trigger:-
Condition: as you like
On activation: "_x setPos getPos GL1" forEach units grp1
Not sure what happens if you teleport them all to the same spot, but you can experiment.