Using the symbol of the first series in the group is an alternative solution but maybe not what is usually expected.

The aim is to provide some group symbol color or icon property.

A temporary fix now allows using the Symbol.OnDraw event to custom paint the symbol.

With the latest version:

procedure TForm1.PaintSymbol(Sender:TObject; Series:TChartSeries;
                             ValueIndex:Integer; R:TRect);
begin
  // Valueindex is the group index

  Chart1.Canvas.Brush.Color:=clYellow;
  Chart1.Canvas.Rectangle(R);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Chart1.Legend.Symbol.OnDraw:=PaintSymbol;
end;