I do have a workaround for you to text multiple people. I need to do this also, and currently the SMS feature only allows one at a time, so you need to find out what the person you are trying to text uses for their cell provider, then a little research to find the carriers email. Example, AT&T still works with the old Cingular email, so if your friends mobile number is 111-222-3333 you would want to add an email address into their address card with the following...
Sending an email to that address will send it to the test message portion of the phone. So, since you CAN email multiple people from the phone, this is how I am currently doing it. ALSO, doing it this way, you can attach photos to the email, and send it to them via text as well.