https://www.theverge.com/2017/10/4/16412256/google-home-update-voice-text-message-support

Here’s a great update to the Google Home: you can now send messages to your Home speakers and have all of them speak it aloud. Google calls the feature “Broadcast,” because it lets you broadcast a message from your phone and out through all of your Home speakers.

To use the feature, you just say, “Hey Google, broadcast ‘What’s everyone up to?’” and your message is supposed to automatically be sent through. It’ll be read aloud in the Assistant’s voice — not your own.