We have a few quasi public channels (see below), and this private email address: team {-ät-} c3sus {-dot-} de
There is a mailing list for c3sus that we use for general internal discussion: Feel free to subscribe and contribute! Mailinglist c3sustainability
There is a Rocket.Chat instance for CCC events. c3sus has a channel: https://rocket.events.ccc.de/channel/c3sus