The Storygraph is a Goodreads type site/app. I've used it a bit, and like it so far. Their business idea seems to be to make money from selling premium memberships, so their priority is the end users. (I suppose everyone has heard the adage about free services: "If you're not paying, you're not the customer, you're the product.") I like the functionality and user interface, and the book recommendations are good, so I'll probably pay to help keep the site alive, even though all the features I'm interested in are available in the free version so far.
They have a new feature, buddy reads, where you can invite up to four other users to read a book with you. You pick a book and get suggestions for people to invite, both people who have this book in their to-read-pile, and people picked by the site's predictive algorithm. You can specify if you're OK with getting invitations from strangers. (It's also possible to pick users first, and ask the algorithm to suggest books, but that's only available to paying users.)
While reading, you can see how far the others have read. When you enter comments, you mark them with how far you've got in the book, and the comments are locked for the other readers until they've come to that point in the book.
I've just tried it out, although I felt a bit self-conscious about inviting strangers to a read-along out of the blue. Of the four I invited, two have accepted so far. One of them had finished the book before I started

and the other hasn't started yet. (I chose a novella so the commitment wouldn't be too big for a first try.)
We have exchanged a few comments, and I like the way it works. When I started reading, I could see that there was a comment waiting for me at 65% of the book. For books which are long and difficult to read, or for when I'm in a reading slump, this might help encourage me to continue reading. It's also nice to be able to comment on dramatic things that happen without having to worry about spoiling people.
Unlike the reading club here it's completely closed, I'm pretty sure only the participants can read it. That can be both a drawback and a good thing, I guess. The limit of four participants is a drawback, but they are working on a book club feature which will allow for unlimited participants.
All in all, I like this feature, and will probably try it again. If anyone wants to interact with me there, I'm hildea there as well. Feel free to invite me to buddy reads