By James Philip
To restore the elements to the group instance, the geometry must be rectified first. Then select the affected group instance. See fig 2.
The missing item reappears as a ghost item. On the ribbon, you now have the option to restore all excluded items. Be careful because if any items were manually excluded these will be restored also. See fig 3.
After option is selected all the items excluded from the groups reappear. See fig 4.