You can make group calls in Viber on your computer with Viber for Desktop
Before starting a group call or voice call, make sure your headphones and speakers are connected to your computer and working.
To make a group call from an existing Viber group
Click the phone button at the top of the screen from inside a group of up to 20 people. Viber has the maximum limit of only up to 20 people in a group call.
To add more people to an in-progress group call
Click the + button to add another person to the call
To initiate a group call from your contacts
From the compose screen, select the people you want to call from the contact list, and click Start Call.