Open hangouts and press the + to send a new text. Use your carrier number (and not your google voice number). Choose a recipient. If it's a contact with no text history, there's an "anyone else?" choice at the top. If you have previously texted this recipient, it will pull up the last thread with them. At this point, click the three dots on the upper right that give you a menu and you'll see the option "new group MMS". Click that and you can add contacts. It will remember the group for next time.