1. There are 3 main skill level groups with NESBA - Beginner, Intermediate, Advanced. There is also an Intro program that will give you a one time deal to see what the track is like. The intro program has less strict tech rules, is free, and rides for the first two sessions with the beginner group. Intro fills up fast because it's free but alot of people doing Intro do not show up because they're not loosing anything.
2. There is a formula used to figure out how many max riders will be at each track. It uses something with the length of the track( I honestly do not know). That is the Maximum number of members that can be signed up for each group for the day. Intro has very few spots when compared to the Beginner group. When the members with the class "B" have signed up and the maximum number is reached, the event (for that class) shows as full. As the event gets closer and people take a closer look at their schedule, some do cancel out. If you have a blackberry or something that will alert you when you have an E-mail I highly suggest it for this. You can change your E-mail address as many times as you want in the system to get the notice that the event has a spot open. For example, I used to change mine to a work email when I got to the office then back to personal when i got home. You getting the spot depends on you being the first to sign up once that E-mail is sent. It gets sent to everyone on the waiting like for that class.
3. I do not recommend showing up to a day that you know is full and waiting for someone to crash out. That person paid for that spot and it will be theirs for the whole day, regardless of what happened.
4. If you pay for the NESBA membership, and are wanting to do a full day, you need to register as a B group member. The intro class is a one time thing and will only get you the first 2 sessions of the day.
Hope this helped you out a bit..