I’m having issues doing this. When I reply to the email on my son’s phone, it signs him into ST, but there’s no indication in the app of what to do next. Has the software changed since this?
If you are setting up a presence sensor for his phone - on his phone, open the ST app and go to Marketplace: Things: Sensors: Presence Sensors: Mobile Phone
Weird, it wasn’t letting me log in before…now it is. F**king computers. 
I have a weird issue. I sent a invite to my daughter, I registered her. When trying to launch the app, SmartThings Classic from her it thinks it’s setting up a new hub and asks for the code. What did I miss ?