Google Hangouts client/app which supports group chats

On OS X Yosemite, I'm using Messages.app to chat with my contacts.

But I can not participate or read group chats from Hangouts within Messages.app, for that I have to use the Gmail/G+ page.

So, anyone knows a client/app for OS X (or even an extension/plugin for Messages.app) which enables me to chat one-on-one and even on group chats?