Thanks for the lead!

It looks like the Buddy Read feature does in fact start with a specific book and organize a group around it, but it invites me to specify all the people that will ever be in the group right away, at group creation time. I get three ways to invite people:

• "Machine-learning powered reading buddy recommendations" - Unspecified voodoo. Three users are shown.
• "Community members who have this book on their radar" - Probably folks that have this on their public 'to read' list. Three users are shown.
• Specifying users directly by username

This doesn't quite fit the "I'm up for this, let me know when it starts" mechanic.

I could create a new group & invite all three of the users with this book in their public 'to read' list, but I think folks treat the the 'to read' list very, very casually -- not at the "I'm ready to commit to a reading group" level. These three users have 723, 2749, and 3771 books on their 'to read' lists respectively. I see that I somehow have have 46 books on mine, & haven't been thinking of it as a 'ready to commit to reading group' list.