Layer grouping should be possible in ArcGIS Online. When adding several layers to the map, the user should be able to group any of them. Just like in ArcMap.
This idea has been merged with Group layers for ArcGIS online.
This would be useful, for example, when adding several shapefiles in one zip file (because you can only add shapefiles with max 1000 features and often you have datasets that have more than 1000 features).