The whole discussion revolves around a fundamental principal that is simply broken to begin with, akin to "How to try not to die when you eat rotting meat".
Don't eat rotting meat. Use a fridge. Etc.
In the case of passwords-
-Use a shared authentication platform -or on sign-up implore that your users do not use a shared password. Education -or offer, or force, a generated password
But instead we'll discuss the risk that shared passwords get lost, when they were in the wild the moment you used them on a second site.