How does this work?

I have been trying this program which works great: https://github.com/erkstruwe/chromecast-cli

One thing it misses is being able to cast to multiple devices. I have been pointed to this project to be able to cast to multiple devices: https://github.com/thibauts/node-castv2-client/wiki/How-to-discover-&-update-audio-groups. Can anyone tell me how it works and how to find the cast group?