The Backdrop CMS community uses a handful of tools for communication.
Issue Queue: Github
The primary means of communication is the Backdrop Issue Queue, which is where all discussion happens around adding new features or fixing bugs.
Backdrop has an official public Gitter channel that pairs nicely with the issue queue. Drop in and say hi at https://gitter.im/backdrop/backdrop-issues.
Backdrop has several recurring meetings on Thursdays you should feel free to join or watch at any time. There is a weekly core developer meeting, a fortnightly design meeting and a fortnightly outreach meeting. See our Google calendar for the schedule: [iCal feed]
Backdrop holds weekly meetings on Thursdays at 12noon and 1pm Pacific Time. These are open meetings that anyone can join, follow us on Google Plus to join in.
Video archives: YouTube
All weekly meetings are recorded and published to our YouTube channel. This channel is also home to tutorial videos on contributing to and using Backdrop CMS.
Discuss with the Backdrop community at the Backdrop CMS Subreddit.
Backdrop has an official Twitter account at @backdropcms.
Backdrop has an official Facebook page at http://www.facebook.com/backdropcms.
Backdrop has an official LinkedIn group at https://www.linkedin.com/groups/6930143.