This is a bit of a judgement call, as there are pros and cons to both options.
Pre-structuring has the benefit of giving your contributors a foundation to build upon, but some people may find it creatively constricting and, as a result, might not add their best stories and photos. If you decide not to add content when you send out your initial invitations and your contributors arrive at an empty project, they may be nervous to be the first one to dive in.
Ideally, you should try to find the happy medium between structured content and creative freedom. Try to encourage your friends and family to create their own chapters, write stories, upload photos, and comment on other people’s content—whatever works best for them. It’s always better to have too much content than not enough, and you can always re-organize the content if needed.