Helmets are REQUIRED.
Helmets must be an open face or full face style, DOT-approved motorsports helmets. Bike helmets are not allowed. Half helmets are not allowed. If your helmet makes anyone think that you remind them of Dale Earnhardt, it's not allowed. Oh, and bike helmets are not allowed.
Your helmet is probably the single most important piece of safety equipment that you wear during an event. Because of this, if you show up with an inappropriate or inadequate helmet, you will not be allowed to race. Most teams bring spares, so in an emergency you can borrow one from them. But don't count on it. A helmet that's good enough for this racing series won't set you back much. We've pulled out a few examples down below.
Also, if your helmet is not properly secure (you know, with a strap) expect the Grim Reaper to penalize you.
And on a personal note, you might want your own helmet instead of sharing, because otherwise you are sharing your teammate's sweat when it's 95 degrees in Detroit in July. (At a minimum two helmets are useful for the Endurance Race so you aren't trying to change helmets while also changing drivers and batteries.)