Sean and I have been looking at Slack integrations for WordPress for a couple of projects. Most integrations go from WordPress to Slack, e.g., when you publish a post to WP, something is sent to a Slack channel.
However, we also found an integration that brings Slack into WP (for a cost). One possible use would be if you're running a course on WP but want to do discussions in Slack, this would keep the students from having to navigate between two interfaces. This does not make Slack channels public. Users still have to log in to see the channels that they need to see (at least, that's my understanding).