Last year when I went, they allowed "stand ins" to attend, standing at the back of the room they had no Skyview to play with, but probably learned just as much. You might try for that. They only have so many desktop units, therefore the limits.
What Don said. We don't have any official sign-up for that, and I'm not sure if the space accommodates extras this year, but I'll try to get an answer on that from Kirk (who teaches the classes, but is out of the office at the moment)
Drop-ins are OK, but it'll be limited to maybe 4-5 people per class. First come, first served basically. You might check with Kirk at the booth the week of before he's off to teach.