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 > 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 > Contacts> Select the Arrow to view the full contact card > Send (in the Communications section) > Select the SMS Icon.

In the App, swipe to the right on a Contact that you would like to call, this also applies to sending an SMS or email. 

The same function is available through the jobs tab on the app, again by swiping to the right.