As the creator of the group, the teacher is automatically included in any message sent to the entire group. If a student wanted to comment to everyone, he or she knew that I would be among the first to read it. In order to facilitate discussions, I would post questions and the students would respond back with their opinions or answers.
Students were permitted to send out a message to the entire group ONLY if they felt it was worthwhile enough to take up everyone's time. Ideally, this included posts like "What did you think about chapter 6?" or "I couldn't imagine living in a concentration camp" which other students took the time to respond to. In a few instances, students did post off-topic comments like "Hey Everyboddddy!!!" - which were scorned by their classmates and eventually disappeared over time.
Students could also use the messaging to send questions to me directly. Occasionally, a student would message me during our lab time rather than walking across the room to talk to me. Most of the messages, however, were sent from home while the students were working on homework and came up with a question that they wanted answered. Because we are on a block A/B schedule, I could answer their question quickly rather than waiting for two days later when they came back to class.