I use Acrobat.com with my clients--would work great for this too. Gives you bi-directional webcam, shared notes, text chat, multiple participants, etc. You can share your screen, or they can share theirs, and you can request to take control. Pretty handy.

I've been doing this for years, came up with the idea in college. It's a great drinking game to bet on the least number of jumps from one obscure article to another.