How can I send a group sms using my google voice number? I have tried using the google voice app only to find that it doesn't support group SMS, and instead sends an individual text message to each person (which doesn't allow them to reply-all to the group I originally messages). 

I have also tried using hangouts, only to discover that it does NOT use the google voice number.

Is there any way to combine these two features? can I do it from a web-browser interface somehow? 

P.S. I would like to avoid signing up for Google+ if I can avoid it.
Google Voice (the service) doesn't support this. Until Google adds this feature there's no way to do it.

This limitation is not specifically noted anywhere official, to my knowledge. However, Voice receives group messages from supported carriers the same way it sends them - as independent messages - and this has been officially stated. Mayur Kamat (the Hangouts product manager) noted this caveat on a Google+ announcement regarding expanded MMS support for Voice:


  Group MMS will arrive as individual messages - so you will at least
  not miss messages any more.


The above also requires that the message was sent by one of the carriers Google supports MMS from. Group messages received from unsupported carriers are never delivered.