ADVICE REQUEST: Internal communication plugin

I have an orthodontist client who wants to create an internal communication system on his site so that he can communicate with dentist that refer him business. The idea is that the dentist can create a login and the ortho and then dentist can then communicate, securely, about the patients they have in common.

I looked at:

Was wondering if anyone had insight as to which they think would be more beneficial for him to be able to communicate with the doctors. I'm leaning toward Communities, because he could create one community for each dentist, and then create threads for each patient.

Or ... if there is another plugin you recommend, please let me know.

Any/all opinions are appreciated.