Your SMS setting is found in Main settings: (Home / Settings).
Once you turn this on, current members will have the option to opt in to text reminders from their member dashboard. All they need to do is log in to their member dashboard, click on the "settings" tab, and then click on "Change Contact Info and mailing address".
Once they save, they will receive a text asking them to confirm they wanted to receive messages related to their subscription. They have to confirm in order to receive text messages.
For new members, they will be asked automatically if they want to receive text reminders or not. And they will text back to confirm or deny, in the same manner as above.
You can review the language in the messages that are sent out automatically. These are the two messages that can be sent as SMS instead of email, you can edit both in the boxes next to the "SMS" options. All three SMS reminders will include a link to the member's box page.
You will be able to see any and all messages that are sent or received by going to Home / SMS Messages. You can also view this on the individual member, by going to their subscriber page and clicking "SMS Messages" on the bottom of the left menu. Furthermore, you can also send individual texts to member's from this page.
At this time, you cannot send out mass texts in the same way you send out mass emails.