Summarize this article:

Once Viber has been downloaded, tapping on the app will begin the setup process.  Input your device’s telephone number and grant access to your contact list, then you’ll be sent a text message with an access code. You’re all set up!  Now you’ll see a number of options at the bottom of the app including Messages, Recents, Contacts and Keypad. Tapping on a contact will give two distinct options.  Free Call, and Free Text.  Selecting either of these options will automatically begin a call or a text conversation with that person.

Summary:
Install Viber on your mobile device. Input the access code you receive via text message. Tap the Contacts button to see all of the people in your phone’s contacts who currently use Viber.