To make conference call, you can:
Add participants to the existing call
While on a hike call, you can add more participants by:
Step 1: Tap on the '+' sign on the bottom
Step 2: Add you contacts whom you want to add to the call and tap on 'done'
Step 3: That's it.
From a Group Chat
Step 1: Tap on the three dot menu from the Group Chat
Step 2: Select Group Call
Step 3: The conference call will initiate soon.