Google Hangouts client/app which supports group chats
Try Jitsi for OSX, it supposedly handles voice for Google Hangouts, but to be honest...

The best way I have found to do this is just to use the following Chrome extension called Hangouts, by Google: https://chrome.google.com/webstore/detail/hangouts/nckgahadagoaajjgafhacjanaoiihapd?utm_source=plus 

The thing is that Google Hangouts is no longer a open XMPP/Jabber protocol its sort of a mutant breed of this technology that is becoming more and more closed off providing features across more open traditional Jabber accounts.

The only caveat is that DON'T quit chrome or it dies, my default browser is Firefox fyi. So I run chrome, click the little hangouts bubble, close all the chrome windows and get the cool Hangouts buddy list every time I hover my mouse in the lower right corner of my screen: OSX Yosemite 10.10.4