I hope screen sharing comes soon. They bought Screen Hero a while back, and said they were integrating it.
I'd hoped that Slack buying them would result in a more polished product, but I haven't heard a thing about it since the acquisition.
I heard they got audio calls, and then video calls, working in the main app.
If I were working to integrate Screen Hero with Slack, that's where I'd start.
https://medium.com/@chris_82106/implementing-webrtc-screen-s...
If you are a paid subscriber to Slack, you can add the app to your Slack account for free. You still have to sign up for Screen Hero as well, but at least you can do it.