When you add someone on Viber, he will not get to know as no notification pop up on the screen. He only comes to know when he scrolls down and your name will appear on the contact list. Moreover, after adding when you message him or call him then he will come to know that you have added him/her on the Viber account.
So, it is better to inform by message or call when you add someone on Viber to make communication and to stay connected. In this way, one come to know that other person has added him and want to communicate.