Please check with the following steps to create Chromecast speaker group:
- Make sure that your mobile device or tablet is connected to the same Wi-Fi or linked to the same account as your Chromecast, or speaker or display.
- Open the Google Home app .
- At the top left, tap Add Create speaker group.
- Tap each device that you want to add to the group. A tick will appear next to each device that you select.
- Tap Next enter a name for your group Save.