You're signed in, but this email isn't on the book club's list yet. Ask whoever invited you to add it.
A couple of quick things.
Every book the circle has read, and the one you're in now.
A few words strung together is easy to remember and hard to guess.
Works while the app is open or recently backgrounded — phones suspend closed apps, so this isn't a true closed-app push.
They'll get an email with a secure sign-in link.