How to add new device to family member

Hey gwen1975,

Family sharing would be associated with the Apple ID rather than the device. We'd recommend checking to ensure they are signed into the same Apple ID that is a part of the family group on their new iPhone: Sign in with your Apple ID - Apple Support

"Sign in on your iPhone, iPad, or iPod touch

1. Open the Settings app.
2. Tap Sign in to your [device].
3. 
4. 
   
5. Enter your Apple ID and password.
6. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in."

You can find steps to invite and join a Family Sharing Group here: Set up Family Sharing - Apple Support

Thanks!