You can initiate casting before joining a meeting by selecting the "Cast this meeting" option, or at anytime during a meeting by hitting the three-dot overflow menu in the bottom right and clicking the same option. Google notes that the feature works with the Chromecast Ultra, first and second-gen Chromecast dongles, and Cast-enabled TVs or displays, although performance may vary.
Meet on Chromecast lets users see their fellow participants at a more lifelike size while still using the camera, microphone, and audio from their computer. It will roll out both online and in the app over the next few days. Make sure you're using the latest version of the Chrome browser and Meet app, and ensure your Cast devices are on the latest firmware.