Open the google home app. Google home has groups of speakers that can be cast to.

Google Home Tips 20 Google Home Tricks For Your Smart Assistant
Can t create speaker group google home. Tap each device that you want to add to the group. All i get is sorry cant find home speaker group please make sure the device is turned on and connected to the internet. Tap your speaker group settings choose devices. Youll be able to select the speakers you want before giving the group a name. Then select create speaker group. I have a similar issue.
To create a group simply open the google home app and tap add which youll see in the control dock at the top of the home screen. On that card using the overflow menu you can add or delete speakers from the group as well as delete the group as a whole. If you have not yet set up a home group launch the google home app tap the plus sign at the top left of the home screen and choose create speaker group. Select the speakers you wish to include. Then you can voice control all speakers with one command. Arggghh ffs google it is.
Or two google home or nest minis but you cant mix. A tick will appear next to each device that you select. Correct you dont understand. Tap each device you want. Tap each device you want to add to the group. Because sonos is not able to be added to these groups you end up having to have a sonos device and a google home device in the same room to achieve the dual purpose having assistant linked mic and ability to function as a true speaker within google home.
Its all linked to the account. My speaker group will show in the home app and streaming apps but assistant when asked cant find the group. If you have two google home or nest home smart speakers you can create a leftright stereo pairing even if they arent the same color. Open the google home app. A check will appear next to each device you select. With a google home 30 at best buy speaker you can throw your favorite youtube videos or netflix shows to your television stream your favorite music or listen to the news or to podcastsyou.
Tap your speaker group settings choose devices. It will take 5 10 seconds or so to show up but in your device list in the google home app you will now see a card for your group.