Now I found the problem. You are right - that "groupSelectedUnits"-command really works, but in normal view units are automatically unselected too early when the command is used via radio. In command-view (numpad del) all works great.
_array = groupSelectedUnits player;
if (count _array > 0) then {
_group = creategroup west;
{[_x] join _group;} foreach _array;
player hcsetgroup [_group,""];
};
if (true) exitWith {};