The key, as @steelmarket hinted, is to get everyone to hear their voice. Once people do that, they are much more likely to speak up later. His idea of asking a question that everyone has to answer is a good one.
Here’s some other hints. If you ask questions that don’t have a right answer, it opens up the space for more people to talk. They don’t have to be afraid of getting it right or wrong. If you are asking about readings, and they haven’t read it, there is no chance they will talk, unless they are really confident about faking it. You can get around this by asking more general questions, or theoretical questions where it doesn’t take knowledge of the readings to answer.
Another thing that would help, but could be hard to implement, is to arrange the seats in a circle. Everyone can see everyone else, which, on it’s own, allows more people to talk. Also, you are just one member of the group, instead of the “expert” and this helps folks to open up. Finally, you can also just go around the circle, and get everyone’s response to something. Everyone knows when it’s their turn, and can prepare for it. You must also allow them to say only a little bit, or even to pass, but if too many people pass, you might have to stop that practice, at least, occasionally.
If you can’t make a circle, so as much as you can. Make yourself look like you are on their level. Sit down! Let them argue it out amongst themselves. You sit out. These techniques are to empower them. It mean you lose your power, but that is a good thing, managed correctly. There are ways to subtly manage the conversation, nudging it in other directions, defusing conflict by asking them to identify underlying principles for their points of view, etc.