@ccc I'm not sure that the https://pythonista.slack.com team is related to the app. I may be wrong, but I couldn't find any mention of someone setting up a slack team for the app in the forum, so that URL probably points to a different group of people. (Someone please correct me if I'm wrong.) Someone would need to register a pythonista-app (or something similar) team with slack (since you can't reuse team names and all).
I use slack at work and think it'd work great for the community here, but it is more of a closed system, requiring an invitation to join a team. Of course, anyone who wanted access could request it here and the admins/owners of the slack team (@omz and some other people probably) could send them an invite. I think it can be opened up where anyone can send invitations, but don't quote me on that.