The only way to add multiple recipients is to hit the menu button and select "Add Recipient" until you have everybody added. Please note that if you have a lot of people in your address book that you can only send to specific number of people at a time.
I believe all you need to do is select "Compose SMS Text" then click on your first recipient. Hit the menu key and select ADD Recipient. This can be repeated until you have all the recipients you want. Compose your text and send.
