Can games with free play days be shared with family?
I think you can't really share them, because everyone can anyway play them for free. Might be the case though that you need to have a Stadia account when you want to play them (especially when you plan to play it online for example, then your family members can not all play with one account, they need to have a separate one). But this is only my thinking, you would need to try it out to be sure.