You can send an SMS to your members in much the same way you send a regular message to your members. SMS access is available in all paid plans (see pricing & directions to buy additional SMS packs).
Before sending SMS messages
Make sure you have added the Phone Number field to your registration form and that your members have added their phone numbers.

NOTE
In the US this verbiage is required verbatim- 'Message and data rates may apply' -please ensure to include this in the Phone Number Field Description.
To send an SMS go to the Outreach Center for your group, click Compose then Send SMS:

Choose your recipients and write your message:

You can see the number of SMS messages you've sent to this group at the bottom. Click Send SMS to send your message to your members. They will receive the message on their phones like a normal SMS. You can manage their replies and ongoing conversations in Mobilize.