Have you attempted the solutions presented in this thread?
I figured it out. To send a text to a group that you have created, hit the messaging icon. You will see the prompt to enter the recipient or phone number. Scroll down until you see the "Groups" category. Tap that and the groups that you have created are presented. Select the group that you want to message and you see all that are in that group. There is then the the option to select all.