Hi Moellegaard the answer to your question can be found in an other thread I had on Chromecast and AudioGroups:
OH2 - Chromecast Binding - Audio output on multiple Google Home (Mini) at the same time?
It was not obvious and a bit try and error. I agree the documentation could be updated.
(This is all done on OH 2.2 - would be glad to know it works on OH 2.3 as well.)