Monitor the messages, but intervene only occasionally. You are the master, students the apprentices. This may help equalize participation. It can give a sense of audience, and increase students' awareness of the range of opinions and approaches. It records all messages for future reference and evaluation.
Students are usually reluctant to evaluate each other's work in public.
Informal brief responses seem to cause least stress but still produce communication that can be discussed further in class. You will need to set questions that are based on a topic on which students share knowledge, yet ones that are genuinely open so students can enrich each other's understanding. You are the overseer, students the workers. They may make the most of this opportunity, using the conferencing system to define problems in course material and explore solutions, and to enlarge the range of discussion beyond class constraints. Or they may use it to tease, bully, and waste time.