A friend came to a book discussion with some written comments to contribute. He didn't say anything about them or use them. At the end, he said,"Why did I bother?" I believe that
Deciding what matters about a book
Forming words to express what matters and maybe why
Writing the ideas out
Bringing the ideas to the discussion
all matter and are all valuable.
Our brains, our speech and our writing, especially just "little" notes, can all seem elementary. Sure, doing those kinds of things are often preliminary to making a presentation or presenting a paper or emailing a group. But each of those construction steps is a use of our abilities and each is a good use of our time and effort, regardless of what or who gets involved afterwards.
I have had occasions when a student brings up an earlier class's discussion, often beginning with a remark something like "if I could just return to a topic we discussed a few days ago…" There is nothing wrong with failing to speak at a book discussion but then emailing the group with the list of comments. Some people are worried about the acceptability of such an action. Some people may keep their eye on responses and reactions to bringing up an earlier topic and decide that no contributions afterward or a frown or a joke means they made a mistake socially or politically. But honest, thoughtful reactions to a book or to other comments in the group may fail to elicit further communication just because the late additions motivated valuable thinking and didn't feel peripheral or superfluous.