I'm in one now and have been in a few. The "formalness" varies. Most that I've been to have a "host" that person usually has a few questions that the group answers. Some people use notes, so it wouldn't be weird to bring them, some people bring the book with sticky tabs or highlights, others just show up, sometimes people haven't read or finished the book, it's all OK.

I encourage you to show up and see if the group has a vibe you enjoy. When it comes to meet up I often "try out" groups and just show up to see if I feel like a fit for the group.