It's usually best to ask everyone for a contribution, and you could mention that you would like to hear from those who haven't spoken much (without naming names).
If that doesn't work, you could ask some of the less-talkative people directly for their input, especially if you're expecting them to make a contribution on a certain topic. Another option is to ask each of the participants in turn for comments.
Be careful that you don't make participants anxious about contributing, particularly if they are new to the group and/or the topic is outside their area of expertise.