Is it just your DD and these other two girls, or are they a few more kids? Will this be a group for just the girls, or a mother-daughter thing?
The best thing to do is let them get used to the group and not expect them to say much the first few meetings. Putting them on the spot will make them unhappy and more unwilling to come, they need to feel this group is a safe and comfortable place.
What I find helps with my DS15 who is shy/quite introverted particularly at first is games. But not ones that put him on the spot asking him to talk about himself. Perhaps something to do with each book? When I ran a girl scout group, I found group games did a good job of getting everyone to open up and relax. You may be able to come up with a word/circle game that will work as an opener for each meeting. I'm trying to think of a good game for a book group, maybe it will come to me later in the day, or others have good ideas.
Other suggestions include having the first club meeting a mother/daughter meeting even if that isn't the overall intention. It will give those girls a chance to warm up without feeling overwhelmed, on the other hand those girls might say even less with mom around. These girls might be more comfortable writing things down, and maybe having them write questions they want to discuss on paper rather than out loud. And if this isn't a mother/daughter group, make sure you are a supervisor rather than running this like a teacher. Perhaps assigning each girl to be in charge of running the discussion each time, just don't assign the shy girls before they are ready.
