There is no such thing as "notification only membership". A member of a Google Nest "household" has FULL access to all devices in the Google Nest "home/structure", including adding and removing other home members, including you, and they determine their own notifications. And yes, they would have to install the Google Home app on their phone or mobile device.
Conaso, thanks for reaching out. Modifying the home memberś access is not supported. Our team is working hard to deliver the best experience for our users — I'd suggest you submit feedback about it, here's how. Keep your eye on the Google Nest Community page for any updates.
Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.
Appreciate the help here, MplsCustomer.