We have wanted to add Whatsapp capabilities to In Contact for a long time but sending messages from In Contact would be expensive. Meta (the owner of Whatsapp) charges approximately 73 cents per message for external applications to do this.
Instead In Contact has taken the approach that you rather nominate a device to send messages from and that you create groups on.
You can use In Contact to build up your contact lists via an invite on your newsletter. You could have multiple groups that should align to your groups in the system.
Here is some more info:
Create a Whatsapp groupGet the group linkUsing a Whatsapp templateEditing the newsletter