Ok, a little more detail now that I have the family member’s phone in front of me …
Invitation was sent successfully and user received email.
User has a Samsung S8 and previously created a “Samsung account” on it.
After user has downloaded the SmartThings Mobile app, I tried to select “New to SmartThings” and attempted to create a new account.
SmartThings states that the user’s email is already in use, so I can’t use this method.
I also tried selecting “I’m a SmartThings User”.
When I input the user’s email address, SmartThings states that the user is already signed in to Samsung with an account (same email address), to proceed with this user hit continue.
If you hit continue, I prompts for a SmartThings hub code.
It appears that the “Samsung” account is conflicting with the “SmartThings” account on the phone.
I just found this related forum post as well …
I guess I’ll need to start over with my hub to clear this up?