To be able to send an SMS to a client this feature must be activated on your account first.
In the Web Portal go to CRM > All Contacts > Click on the Contact or Billing Name > Select the Call or SMS Icon.
You can do it via the contact card page as well, CRM > All Contacts > Select the Edit Arrow to view the full contact card > Select the SMS Icon.
In the App, swipe to the right on a Contact that you would like to call, you will be given options to Call, SMS or Email the Contact..
The same function is available through the jobs tab on the app, again by swiping to the right.