Google Hangouts does not support federated communications. This means that users on XMPP-compatible networks cannot communicate with Google Hangouts users.

All you can do for now is to connect to Google Hangouts with an XMPP client, which doesn't mean much, doesn't work so well and given that Hangouts is explicit in its lack of support for XMPP, we don't know for how long this will last.

You mean like group chat? One of the key features of Hangouts?

Oh well, it's Google's loss, we've moved to Slack where I work instead.