iPhone 6 group chat face time

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.

Your iPhone 6 Plus can only do group FaceTime audio call.

You have to upgrade your iPhone to a newer model if you want Group FaceTime.