Is the group you are trying to add only comprised of plugs? I would suspect that group would be different than groups of hue bulbs and therefore might not be able to sync across. If you add a hue bulb to one of your "not-found" groups is Hubitat able to find it then?
Also, you are syncing normal bulbs too, right? There's a minor bug that you have to sync at least one regular bulb and not just groups.
