Is there an option to group sublayers in ArcGIS Online Map Viewer?

Answer

There is currently no option to group sublayers in ArcGIS Online Map Viewer and Map Viewer Classic. Feature layers are grouped with sublayers by default in Map Viewer and the option to group layers is only available in ArcGIS Online Map Viewer.

Alternatively, sublayers in ArcGIS Online can be grouped by editing and saving the web map in ArcGIS Pro, which is described in the following steps.

Note:
This workaround is only supported in ArcGIS Online Map Viewer.

1. In ArcGIS Pro, open the web map.
2. In the Contents pane, hold Ctrl and click each desired sublayer.
3. Right-click the selected sublayers to open the drop-down menu and select Group.

4. Save the web map.

The image below demonstrates a web map with grouped sublayers in ArcGIS Online Map Viewer after the web map is edited and saved in ArcGIS Pro.