Please note that for Group FaceTime video calling, Apple recommends an iPhone 6s or later models, iPad Pro or later, iPad Air 2 or iPad Mini 4 with iOS 12.1. Those who have older iPhone/iPad/i-Touch models that support iOS 12.1 can join in the calls, but only through audio capability.
Now there are two ways you can do Group FaceTime video calling. The steps for the direct way are given ahead.
Go to Settings on your device
Turn on FaceTime and open it
Tap the ‘+’ at the top right of the window
Add the people you want to join in the group call
Tap Audio/Video to make the call
Once the call starts, you can slide up the menu pane for more options that let you add more people to the call, turn off your camera etc.
The second way is using iMessages. So, you can also start a Group FaceTime call from within an iMessage. Here’s how you can do it:
Open the message you’d like to start the video call from
Tap the contacts at the top