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:

• Your account activity does not show that you are part of the same household as the existing members.

I cannot invite my gf to the Families, while we could do Library share before just fine.