Apache supports a Slack. But it is only available for posting by project members. https://cwiki.apache.org/confluence/display/INFRA/Slack+Guest+Invites No members can join into a single channel, but I am not sure exactly how that works. The sign-up is here http://s.apache.org/slack-invite
I created a #nuttx Slack channel in the ASF Slack. My understanding is that anyone can join a single channel even if you are not a project member.
NOTE: The reason that only project members are support is because this is the paid slack which costs somewhere betwee 3 and 4 dollars per person per month. Hmmm... It never asked my any project credentials when I signed up????
Greg