The external sharing function in Dropbox is currently all or nothing. It would be really helpful if you could enable external sharing for specific groups or admins only.
That would be a nice option. At the moment though for "Sharing externally" there is already the option for "On (Team + approved)". It looks like the approved list is then managed via the admin panel only. That way, the admins still control the list of external people that can be invited by your users. That might be a good solution in the meantime?