I'm part of a book club that meets at a bar, it's a pretty relaxed atmosphere. The book club president usually posts questions the day before or day of in our FB group, and at just about every meeting, there is one person who has not finished the book but comes by anyway to see what others think. A few times, I have had to lead book club and I have been the person who has not finished the book lol. Our group is usually small enough that everyone has a chance to answer each question, usually 4-6 questions, and the number of people varies from 2 to 12.

You'll be fine, but if your anxiety is pretty bad, maybe pull the facilitator aside and mention it, and ask that they not call on you for an answer if that's how they run things.