Добавляйте в группу после события ADDED

Код AS3:
private var protectGroup:RadioButtonGroup = new RadioButtonGroup("protectGroup");
private var multiGroup:RadioButtonGroup = new RadioButtonGroup("multiGroup");
public function Editor() {
protectNo_mc.addEventListener(Event.ADDED, addRadio);
protectYes_mc.addEventListener(Event.ADDED, addRadio);
}
private function addRadio(e:Event):void{
protectNo_mc.removeEventListener(Event.ADDED, addRadio);
protectGroup.addRadioButton(e.currentTarget as RadioButton);
trace(protectGroup.selection);
}