It's about 66 euros to join and you and 2 guests get in to the grandstand for free. So if you and your Dad join you can take 4 people in at no extra charge (in to the grandstand, you still need seperate tickets to get into the track). Last year we arrived thursday lunchtime and bought our tickets on the door. If you're going to stay on the campsite opposite the track it's free BUT you need to show the security guards your track ticket.
You can join the club on race day like I did last year but in hindsight I would join as soon as you get there, they have good food, cheap beer, clean toilets and even good showers you can use all free of charge.
Oh yes if you ride down there is somewhere safe to put your crash helmets too.
Must stop typing I'm getting excited at the thought of going back.