cmdCreateShareParamterFamilySystem.cs

public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {

            UIApplication uiapp = commandData.Application;
            UIDocument uidoc = uiapp.ActiveUIDocument;
            Autodesk.Revit.ApplicationServices.Application app = uiapp.Application;
            Document doc = uidoc.Document;
            //code
            Reference r = uidoc.Selection.PickObject(ObjectType.Element);
            Element Element = doc.GetElement(r);
            string praName = "DM_PanelAssignment";
            string groupName = "COMMON";
            Transaction tran = new Transaction(doc);
            tran.Start("Run");
            Parameter Param = GetOrCreateElemSharedParam(Element, praName, groupName, ParameterType.Text, true,true);
            //Element.LookupParameter(praName).Definition.ParameterGroup = BuiltInParameterGroup.PG_DATA;

            MessageBox.Show(Param.Definition.ParameterGroup.ToString());
            tran.Commit();

            return Result.Succeeded;
        }