To use a Message Templates with a group Broadcast Message simply navigate to the Groups icon on the left navigation.
Once in the Groups click into a specific Group and you'll see the "Send Group Broadcast" button.
Also see: How can I create a group?
After clicking the Send Group Broadcast" button you'll be taken to the Broadcast Messages area where you'll notice the "Messages Templates" icon toward the bottom of the section.
Click the "Message Templates" icon to dropdown your saved templates, select a template and click send to Broadcast a Group Message.