Hello! We are excited to announce Steam Families is now available for all users. Steam Families is a collection of new and existing family-related features. It replaces both Steam Family Sharing and Steam Family View, giving you a single location to manage which games your family can access and when they can play. Create a Steam Family To get started, you can create a Steam Family and then invite up to 5 family members.
My guess is this, which is way at the bottom of the support FAQ page (which can be found at the bottom of the posted FAQ section):
“I cannot join a Steam Family”
If you cannot join a Steam Family, it is likely for one of three reasons: