@joollzzzy: Please open the Messages app > Tap the speech bubble in the bottom right corner with the '+' symbol inset > Tap the '+' to the right of the Recipient field, then tap on which contacts you'd like to send the message to. It may be dependent on your network as to how many contacts you can message at once, however I believe the default is 20. With this in mind, you will most likely need to repeat this process a few times in order to reach all your contacts.
