In my experience I have found focus groups to have their share of challenges mentioned above, but they have their advantages too. Good conversations can emerge and many times the tangental ones that happen provide great insight. Encouraging and guiding these conversations is key. I've done this in person, but not via WebEx which adds a layer of complexity to this because you can learn a lot from body language and
I'm not sure how much time you have and how much needs to be reviewed, but you may want to consider allowing users to explore or review the product on their own with a companion guide/script for them to follow. Then have them complete a short survey. Hopefully that will allow your group to form their own opinions and provide direct feedback prior to engaging in a discussion.