I uploaded a geodatabase I created with multiple feature classes to AGOL, then told AGOL to create hosted layers for the feature classes. These hosted layers are all grouped. Is there a way to add a new layer to this group?
That group layer is just at the web map level. In Map Viewer you can drag other layers into or out of the group. You can also use the overflow action on layers (including group layers) to create new groups or move.