A single Office 365 subscription apparently "covers the entire household": "You can each sign-in with your individual Microsoft accounts using your settings and accessing your documents, but you still need just one subscription".
I bought it with my account, hence I guess there should be some way to tell it which other accounts are part of my household, so that they can download it. However, I haven't found anything like that in the web interface. So, how do I install the copies for the rest of the family?