Out of class student communication is always a challenge. There are plenty of options: Piazza Facebook group Slack, Discord, or other chat system Discourse, Vanilla or other discussion forum system Mailing list but all have warts. I shared my thoughts on a number of these options a while ago but thought I'd update them now. Most of my opinions hold form my earlier post. I was using and continue to use a mailing list as I can be pretty sure that students will get the email and they don't have to go to any outside site or application.