Text messages are sent through the messages feature. When you select Messages in the Hub navigation, you will be presented with two options (if hub permissions allow): Announcement and Discussion.
Text Messages are only available through announcements because they are supposed to be short and sweet. When you create an announcement, you can send it as an email and you can copy the first 140 characters in the email to a text. In order for people to receive texts they need to opt in by editing their profile and checking off the "allow texting" box, put in their cell phone number and their service provider.