To do that as part of your model,
you would likly have to build a custom script tool in arcpy
and add it to your model.
Alternitivly, you can run the model from an enclosing script that
slots the output into your mxd.
The arcpy script will use the arcpy.mapping functions to work with the mxd;
particulary AddLayerToGroup (data_frame, target_group_layer, add_layer, {add_position})