click the + in the bottom right....add as many people as you would like to the conversation. If you are in the contacts app and want to create a new group or add someone to a group, edit the contact and at the very bottom is the group options.
I have a similar question. I have an existing group with 13 members in google contacts. I want to be able to send one message to all 13 of them via just sending it to the group. I was able to do this in a few steps on my galaxy note 2. I can't seem to find a good way to do this on my nexus 6. I would prefer to not have to add each person manually every time.
Found and answer to my own question, in case it can help anyone. Just use Textra SMS, which supports sending a text to a group (I'm talking about a "group" in this case as groups defined in Google Contacts).
you don't create the group in textra.
you go into each contact. scroll down and add them to a group or make a new group in the contact and add the rest of the contacts to that one.
then go back to textra and choose the group you have just made.