Slack released Shared Channels several months ago in Beta, and it is now a public function. Basically it ties together two teams via a shared channel request, and allows ALL members of both teams to collaborate in that channel on a project.
We use this function as a marketing agency to add our clients into our slack, as they often already have there own established slack channel. This reduces clutter and manual work for both teams and keeps up focused. We need the ability to use those shared channels in Workast, Right now we can select the channel into Workast and make a space, but it will NOT pull the users from the other channel into the space, only the users in our channel.
We have tried other slack systems for PM like this and they have got it to work as described.