Update: 21-09-16

Quickchat

@A095HET9R-quickchat

Quickchat lets you start a realtime HD video chat instantly from Slack.
There is no application or plugin to download and install. It is the fastest way to start talking to anyone using Slack.
Just use the /quickchat slash command to send your Slack team member a shared chat link. When they click on the link, you can start talking. No need for them to add you to a buddy list or contact list.
NOTE: Please ensure you're using the same email address in both Slack and Quickchat.