I tend to have excellent retention when i read books, so i rarely make independent notes. When I read things for class, I normally highlight, or make notes on the page. You shouldn't worry. I've been part of several book groups, and they tend to be relaxed. No one's going to be testing you on the important parts of the text.

If you're reading a well known book, I also often read cliff or spark notes online before reading (never as a substitution) because having an overview of the text can hep take your mind off of trying to sort out a complicated plot, and instead allow you to focus on more subtle aspects of the book.

Castaneda explains how to.