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.