A guestbook is a page of messages for someone. They can’t read it until you give it to them. Everyone who can contribute can leave a message, a photo or a video on that page. Voice notes need Plus, Together or an active Plus trial, even in a free birthday guestbook. When you start a guestbook you say who it is for. If that is a person, they cannot see it at all (not the book, not the messages, not their own name inside it) until the guestbook is given. Everyone else reads and writes in it in the meantime.
You give a guestbook you started yourself by tapping “Give this to them” when you are ready: for a wedding, a new baby, or simply because it is finished. A guestbook you start for the whole group has no secret to keep and is visible straight away, which suits a shared note of support when somebody is having a hard time.
If the group celebrates birthdays, each shared birthday starts its own guestbook about a week before the day, and gives itself on the morning. Nobody has to remember, and the person having the birthday sees nothing until it arrives. Starting a guestbook by hand is part of Plus; the birthday guestbook is not.
Find Guestbooks from the Plans tab. The person who started one can rename it, stop new messages once it is given, or delete it.