Try placing a unit somewhere on your map, somewhere out of the way......a small island perhaps.
In his init field put:
GroupWun = group this; deletevehicle this
What this does is make this unit a group called GroupWun, then it deletes him.
The point being, the group now exists and you can spawn more units to join this group, because spawned units can only join an existing group otherwise they will not be created.
Something like:
"SoldierWB" createUnit [_pos, GroupWun]
I hope I read your problem correctly.
Planck