You can lock rooms. By doing so you ensure that only people that you as moderator invite are permitted to this specific room. This way you can ensure that a room stays internal only for example.
Markus is correct about the room moderation feature, it will allow only moderator(s) to add/remove participants but not only internal. Sorry, I may have misunderstood your question.
I would like to see something as internal only as well. I see the potential of an accidental add becoming an issue, or adding some external customer one month later and not reading through the previous trail of conversation before doing so. Marking it as internal only would give even the moderator a bit of a line to which an accidental add isnt encountered.
Additionally, adding an "Internal only" option on a post might be useful. Where as if the room setup is internal only all comments posted are internal unless explicitly set to "Allow all to see". anyone with an @mydomain.com address would not see any internal marked communication.