You’re right, address book contacts do appear - I forgot about that because I disabled autocompletion. So the contacts are available, sorry the confusion.
But the two group entities still are different and may confuse your users, that’s true. An alternative would be to enable the registration app and allow those externals to create an own account to have them available for the NC groups. Though the NC admin (you) should check the validity of the created accounts and assign them to the right NC group. Still administrative efforts, as the registration app doesn’t have an integrated “validate user” functionality yet, so you must do that manually and delete or disable those new “invalid” users. Such an integrated validation feature (notification and a “validate” button in the user management) would be awesome, I’m waiting for a quite long time for that one 