Private messaging through the site

Hi, I have therapists at securetherapy.com - each one is securetherapy.com/therapistname - when a client comes to ask a question, I want to keep them on the site, because email is not HIPAA compliant (I need to be HIPAA compliant for online therapy). Is there a way that I can have them communicate online, through their site preferably, or even kind of like this - is this a plugin?