I believe ATP is not similar to regular sign ups. There is a smaller, limited amount of spots and an equally limited amount of instructors that teach ATP. I wouldn't personally take that chance. Additionally, I would think the instructors would like to know in advance who will be signing up. Not sure if that fully answered your question, and someone might roll in here and clarify. Is there a reason for the hesitation?
The ATP is indeed for smaller groups of riders (typically 3 per each rider group: novice, intermediate and advanced). We do need a little prep time and to make sure we have coaches allocated for the students that sign up. It's typically just a few student per instructor all day long. The event is full and if you try to complete sign-up for ATP, it will remind you that it's full.
As much as people want to ride, riders always have "something" that comes up with work or other personal circumstances. I'd suggest just getting on the wait list and jump the instant you get a notification that a spot has opened up.
PittRace has become a very popular track and is usually selling out weeks in advance.