Um… I pay for screens. As long as my account is within that screen limit it shouldn’t matter who is using it.
This has been talked about for a while now, and I've never seen an explanation of what 'the rules' actually are - it's implied that it's supposed to be per 'household' (but obviously travelling is allowed) but then it says some of this is 'legitimate' being between 'family members'; so two related households are fine?
Honestly, it should just be that it's limited to people you're comfortable sharing the password with (total control of the account!) and the number of screens on your plan, as it is. I used to share with family and upgraded the plan when we used it more and found two concurrent streams wasn't enough.