I have been trying out discussion boards for my students. The results so far are rather mixed, but it may be helpful to others to summarize the position. I would be very interested to hear from anyone else who has tried the idea out.
Discussion boards go under various names.
Software for doing this is widely available. You would need to check what support exists in your institution. It is possible to restrict access but I haven't found any need to do so. The risk of an outsider posting messages about my economics courses is small, and I have no objection to outsiders reading about the course. I can delete any inappropriate messages but have rarely needed to do so.