I think there is an official join fee but there are several ways to join without paying a fee.
Basically you give them all your info (driver's license etc) for your account and then when you rent from them you don't have to stop at the counter. You just go out to the garage, choose a car from the Emerald Club aisle, and then give your information to the guy at the booth when you drive out. Sometimes there are upgraded cars in the aisles - like you go to the Full Size aisle and there will be a mini-van parked there. If it's in your aisle then you can take the upgraded car.
There's a discount code attached to being an Emerald Club member but I can usually find better discounts by searching online by searching online for other codes.
The only downside of the program is you are trusting National Car with your personal info to store in a computer database so there are risks and concerns there. But they're going to get your driver's license and credit card once you rent the car.