I did this last year. Very hard indeed!
Entries are open from December I believe, although you can enter right up until the day before.
Web is
http://www.sportcommunication.com/GT/epreuve2.php?langue=2&trophee=128
Riders set off in one of three groups depending on your registration number (sign up early and get a good number). 1st group of 2,500 sets off at 7am, next 2,500 at 7.20, with all others at 7.40 or 7.50. Organisation is very good, with the road over the glandon closed to other traffic, and feed stations are also good with plenty of variety. Scenery of course, is spectacular, but not wholly appreciated once you've been climbing for several hours
It's the best (and hardest) ride I've done to date.