Did you follow the steps mentioned in this article to start a group call on FaceTime?
Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support
Does your device meet the hardware and software requirements?
...
To use Group FaceTime video calls, you need iOS 12.1.4 or later, or iPadOS. on one of these devices: iPhone 6s or later, iPad Pro or later, iPad Air 2 or later, iPad mini 4 or later, iPad (5th generation) or later, or iPod touch (7th generation). Earlier models of iPhone, iPad, and iPod touch that support iOS 12.1.4 can join Group FaceTime calls as audio participants.
...
copied from: Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support
Any restrictions set up in the settings for Screen Time?