Ok. Had this problem myself. Here's how to fix it.
Start with 'People' and then move the bottom slider right to 'Groups'. Now select your group, and again, move the bottom slider right to 'Group action'. Select 'Send group message'. Now input your text and all will send as a text message. Still a max of ten though
If you tap the top bar with the names in, before you compose your message, the names of each individual will display in the next field. Tap on a name and you will see the mobile number for that person below the name. If you send a group message via 'messages' this will often display an e-mail address, hence the mms or picture message charge when you send the message.
Ok, so what if you want to forward a received message, like a joke that you want to share with your friends? Tap 'Messages' and select the contact thread with the message you wish to forward. Press and hold your finger on the message. Now tap 'Forward' and the message will be displayed above the keyboard. Press and hold your finger on the message and a box comes up with 'Copy all'. Select this and then press back to the home screen. Starting with 'People' again, go through the process above for a group message until you are ready to compose your message. Hold your finger on the 'Tap to Compose' box above the keyboard and select 'Paste'. Your message is now primed for sending to your group and to mobile numbers only.
Hope this helps and although it's a pain, at least it's better than adding recipients individually.
Let me know how you get on,
Dave.