Once Viber is able to access your address book you can not only add new contacts but delete unwanted ones and edit their information from within the application.
Phone contacts automatically appear within Viber and with those who own Viber, an indicating symbol of Viber logo appears next to their name.
In case your contacts are not getting synced, you can:
• Ensure you viber is the latest version. If it’s not the latest, download/update to the latest one.
• Refresh your contact list. Pulling down on your touch screen and release.
• In your device settings, make sure that to allow all contacts to allow Viber access to your contact list.
• You can delete and re-add a contact.
• Try restarting your phone.