In an ideal world, anyone who can access the HomePod physically _should_ have the ability to contribute items to the shopping list, regardless of whether that list is being shared among multiple users or not. At present, in order to add items to a shared "shopping list" in Reminders, each individual must authenticate themselves through voice recognition on their personal iOS device. It's not possible to add items to the "shopping list" without this initial authentication. Additionally, enabling personal requests is a prerequisite for adding items to a reminders list on HomePod devices. Even when it's set up correctly, it takes nearly a 20 second lag to add things to a list. With Alexa, it's instant without the need for authentication.