Try this:

• Create a new text message
• Select the contacts icon next to address field.
• Select multiple recipients. (You can also Swipe left to Groups and select a number of contact groups)
• Uncheck the box for "Send as individual messages. (only required the first time)
• Acknowledge "Group messages" dialog
• Enter or acknowledge phone number for Group Messaging
• Type your message and send

How can I send a group message with the HTC First phone. I've created my group, but can't find out how to send a group message.