Yes you can. In an ongoing 1-on-1 call, you can add additional 3 more people as a group call. You don't need to create a create a group chat for this. While a 1-on-1 call is on, you can just tap the “Add to Call” button and choose the users from the contact list that you’d want to add to your call. You can always simply type the number of an user directly if the user is not in the contact list.
