I've been to lots, and now teach one day beginner clinics. Here are my thoughts.
-First, if you aren't clear about things like suggested footwear or if your bike is set up correctly, ask ahead. Your coaches will appreciate it.
-Be prepared for a long day outdoors in the sun. Keep yourself well hydrated and fueled. Working on skills can take an incredible amount of energy, both mental and physical. You might be surprised at how much energy it takes, even if it seems like you are standing around a lot. Being well fueled helps to fight anxiety and fear, also.
-If they want you to use flat shoes, have something suitable. We had someone show up with "five finger" shoes when we requested they wear shoes for flats. <head bang>
-Can you lower your saddle? If you do not have a dropper post, make sure you have a quick release and your seat post has some room to move. 1 or 2 inches is not enough. I would think with DH bikes this is a non-issue but I could be wrong.
-Have an open mind. Be willing to try new things, even if it feels wrong or different. Breathe through anxiety: don't be afraid to take a minute if you need to.
I was always taking notes during breaks, lunch, and afterwards; that helps my learning. If your friends are going, it will be really fun together afterwards and help each other with practice.
have fun!!