Please check with the following steps to create Chromecast speaker group:

  1. 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.
  2. Open the Google Home app .
  3. At the top left, tap Add and then Create speaker group.
  4. Tap each device that you want to add to the group. A tick Check will appear next to each device that you select.
  5. Tap Next  enter a name for your group Save.