Along with its new design, and speed improvements, Hangouts 4.0 will bring along with it the ability for Google Voice users to participate in group SMS (sometimes referred to as group MMS) conversations. While there are a few catches, this is a huge step in the right direction for Google Voice users.
First off, the new functionality won't be available until Hangouts 4.0 is released to everyone, and since Google generally stages its roll outs, that could be a few days out still. Secondly, you won't be able to start new group conversations, but you will be able to receive and reply to them. (Baby steps.) This is also applicable for those using Project Fi, who until now could only participate in group SMS with the built-in Google Messenger app and not Hangouts.
Once the feature is enabled, you'll not only be able to receive and reply to group SMS from Hangouts on your Android phone, but also from an iOS device, or the web, so long as you've integrated Google Voice with Hangouts. Be on the lookout over the next couple of days for the new feature to go live!
Source: +Alex Wiesen