Here's a bookmarklet I created for selecting a random attendee of a Meetup. I use it for giving away copies of my book after giving a talk.
To use it, click and drag and drop the below link onto your toolbar. Once you're on a meetup event page you can click the link in your bookmark bar to activate it. The page will then fade out the non-chosen members and scroll to the chosen member.
Once you run the bookmarklet, you should see something like this:
The bookmarklet can be clicked multiple times in case the selected member is a no-show.