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.
Honestly sounds awesome. I've found that I love top down city builders (shout out to timberborn) and I really like the theme and setting of Against the storm. Love me some grim vibes in games. Thanks for the info!