To send a Group Broadcast Message, simply click on the "Groups" icon in the left sidebar, then select your group (or create a new group by clicking the plus button).
Once you're ready to send the message, click Send Group Broadcast at the top of the Group details. This will automatically bring you to the area where you can customize your message and send!
Reminder: sending a Broadcast message will send out the message to each of the contacts in the group and create one-on-one conversations with each. When a member of the group responds, their response only comes back to you, not to everyone else in the group!