Mattermost is a team-chat service (a free and open source alternative to e.g. slack.com, discordapp.com, teams.microsoft.com ). You might use this to chat generally with students, faculty, and alumni of JMUCS, with clubs or other special interests (such as professors' research labs), or to chat with TAs for help with coursework.
Find a quick tour here https://www.youtube.com/watch?v=8uBzF1K9Sf0
You can sign in with your JMU EID by:
In Mattermost, channels can be public or private. A “channel” is a group chat that certain people may wish to join (unlike the “Town Square” channel which has EVERYONE in it). See more about channels on the mattermost documentation.
You can customize the color scheme of Mattermost, your notification preferences (on browser, desktop, and mobile), and even your nickname.