What about using a forum for this, a private forum? Do you have a website? If so, your hosting service may offer one or more free hosting software packages that can easily be installed by logging onto your web server account and installing it from a control panel.
As administrator, you can limit access to registered users who must be logged on the forum with their username and password before they can not only post messages, but even to view the forum. In other words, the contents of the forum would not be open to public viewing, as is the case with forums like computerhope.com; only logged on members could view messages.
If you need to use the web for communicating about cases with numerous other parties, the forum approach would be much easier to manage, in my opinion, since you would not need to set up some type of "conferencing" software with each user.