You will not need a huge amount of preparation for this rally. Knowing how to drive a 4×4 will be an advantage but you will quickly learn while driving. The organisers will be there on the terrain to explain how to use the long and short gears which are mostly necessary when driving in sand. There will be some driving in the sand if not in the dunes themselves. We are in the Moroccan desert after all! My recommendation when hitting the sand is to accelerate to avoid sinking the car. If you must slow down, change gear but do not brake.
The participants should learn to use their compass before the rally. Most smartphones have compasses nowadays but if yours doesn’t then you need to buy a compass (Amazon has a big choice of them. A simple one is all you need).
Physically, you need to be in good shape as there might be some walking (especially for the navigator ) but there is no prior training necessary.
And take a good camera/gopro with you. The sceneries will be breathtaking. The atmosphere around the camp fire will be great. And if you’re an instagram fan, photos of the food prepared in the desert will be the best.