Do you realize that this ride happened two years ago? However, there will be more. The next organized ride around Lake Geneva is the Cyclotour event, in which over 2,000 people participated last year, more than half of whom did the full distance which starts and finishes in Lausanne. This year's event will take place on May 31st, 2009,
see here for full details. The people doing it have quite a range of abilities, so you'll likely find a decent group to stick with.
Later in the summer, maybe late August or early September, then SamCole may organize another "ice cream tour" version since he's done so the past two years. He's already mentioned it to a few people this year. That event is a pretty relaxed pace of around 25 kph (not including the multiple stops) and everyone pretty much sticks together; it's the distance that makes it a challenge, not the pace, but the ice cream along the way helps with the stamina.
If you post a dedicated thread then you may be able to find some other people in Geneva who want to get together to ride.