Natively you need to enter group members one by one... Easy Group is a 3rd-party app created for this group texting feature. It is free.
Easy Group on App Store : http://itunes.apple.com/us/app/easy-group/id461469079?mt=8&uo=4
Note: As I am Easy Group developper, I may receive some form of compensation, financial or otherwise,from my recommendation or link.
If you already created a group list, you go to the new message and tap the name of the group and then tap "Enter" in your keyboard. Write down the message and just send.
Hope it will be helpful
Currently Being ModeratedNov 5, 2012 12:02 PM (in response to Kaka Dongguan)
Kaka, did you try the solution you are proposing?
Unfortunately, you can't select a group as text recipient. When you tap on the group, it will not select the group, it will display group members....