Thanks for the replies.
You raise good points. Regarding payment I would suggest one would pay the anual fee, assuming higher financial risk but assuming the administrator role. The other users would pay and keep access on a monthly basis.
I don’t know how different accounts are managed in these plans. I assumed their space was independent and they wouldn’t be able to mess with each other data (aside from the administrator maybe), but maybe that isn’t the case... I can’t see a way around that problem. Even with close friends.