iPhone - How to add people to on-going video chat in Facebook messenger?
I got it. It's not the iPhone's fault, it's the fault of the other phone which uses Messenger Lite. Messenger Lite doesn't have conference call feature (at this moment). So if Messenger (full version) calls Messenger Lite, the "add people" button isn't available. So I install Messenger (full version) for the other phone and try calling again, and now I can see the "add people" button.