KOMASO offers the perfect integration with Slack. Any standard user on your team can start a free call for up to 4 people in any channel by using the simple "/komaso" command. In response a call web link (URL) is posted in the channel. No account login, no password, or name or anything is needed to join a call. To invite users not on Slack simply send them the URL via text or email.
And screen sharing is included. From the browser with no software to install. A one time (10 seconds) plugin in your Chrome or Firefox browser is all you need. And anyone on the call can share.
KOMASO is free for up to 4 users on a call. Unlimited calls, even many at the time in all your channels.
To set up and allow any standard user in your account to make HD audio/video/screen sharing calls, just click the icon at the top of the page. You will be prompted for credentials by Slack to authorize KOMASO. And you are done.