Can I use one Diary for multiple people’s records–say, myself and my child? If so, how many people at most?

Each Diary is designed to be used for one person’s health data. However, there is no limit to the number of people who can be invited to view or edit someone’s Diary. You can add Guests, or view the Diaries other Owners are sharing with you, via your Accounts tab. Tap the torso icon at top right, and add guests via the plus button to the right of the words, GUESTS TO MY DIARY.