To enable:

1. In a premium event, go to Setup -> Registration Form -> Registrant Information.
2. Edit the Cell Phone field.
3. Mark it as visible for your event registrants.

Now your attendees can register with their cell phone and receive communications from you.

To send messages:

1. Go to Promotion -> Registrant SMS.
2. Schedule a new delivery.
3. Either pick a message you made previously or create a new SMS Message.
4. Pick who will receive it and when you want it sent.