The Dialogflow Hangouts integration lets you create bots you can include in one-on-one chats as well as chat rooms. From the Integrations menu on the left, you can enable this feature and select your agent environment.
You can follow this guide for more information: https://cloud.google.com/dialogflow/docs/integrations/hangouts
But you are probably here, to figure out how you can integrate this with rich messages such as cards, formatted texts, images or custom payloads. Maybe you are even trying to enable this via your fulfillment code. Well I got you covered!