You can email us to get a link to our Mattermost channel and eventually a signup link if you're not on Extinction Rebellion's Mattermost instance yet.
The Mattermost server is self-hosted by Extinction Rebellion so your data is not being held by a giant multinational. Sign ups are not strictly regulated however, so unless you're in a private channel (indicated by the lock) assume that police / your gran (whichever scares you most) can read all your messages. For more sensitive information we will tend to use Signal chats.
The channel linked above is our "Townsquare" where we make announcements. We also have other channels, which you can find by clicking on plus icon in the top left beside "SR" and above "Find channel" and searching "SRBE" (all our channels start with SRBE since we use the global SR team).
If you've used Slack or Discord the rest should feel quite natural to you. In particular feel free to spam the "Threads" feature by replying to messages rather than writing to the channel directly.
If you're less familiar with Mattermost, the first time you open the desktop application you should be presented with a tutorial, so follow it! You can also check out the documentation on Channels which is probably all you'll ever need.
Yes! Click on the gear icon in the top right corner to get to Settings ---> Notifications ---> Email Notifications. Unfortunately there are not many customisation options, so you will be notified whenever someone messages you or tags you.
Ask in Mattermost help.