public class SubtypeGroupLayerCreationParams : LayerCreationParams 

var subtypeGroupLayerCreateParam = new SubtypeGroupLayerCreationParams
(
    new Uri(@"c:\data\SubtypeAndDomain.gdb\Fittings")
);

// Define Subtype layers
subtypeGroupLayerCreateParam.SubtypeLayers = new List<SubtypeFeatureLayerCreationParams>()
{
  //define first subtype layer with unique value renderer
  new SubtypeFeatureLayerCreationParams()
  {
    SubtypeId = 1,
    RendererDefinition = new UniqueValueRendererDefinition(new string[] { "type" })
  },

  //define second subtype layer with simple symbol renderer
  new SubtypeFeatureLayerCreationParams()
  {
    SubtypeId = 2,
    RendererDefinition = new SimpleRendererDefinition()
    {
        SymbolTemplate = SymbolFactory.Instance.ConstructPointSymbol(
          CIMColor.CreateRGBColor(255, 0, 0), 8, SimpleMarkerStyle.Hexagon).MakeSymbolReference()
    }
  }
};

// Define additional parameters
subtypeGroupLayerCreateParam.DefinitionFilter = new CIMDefinitionFilter()
{
  Name = "IsActive",
  DefinitionExpression = "Enabled = 1"
};
subtypeGroupLayerCreateParam.IsVisible = true;
subtypeGroupLayerCreateParam.MinimumScale = 50000;

SubtypeGroupLayer subtypeGroupLayer2 = LayerFactory.Instance.CreateLayer<SubtypeGroupLayer>(
              subtypeGroupLayerCreateParam, MapView.Active.Map);