If you aren't able to make it up for the two days you can always register the morning of the hike also but will say this many of the hikers get on the trail earlier then the registration would be set up in the morning. Also, many of the hikers tend to stay at the area camp grounds and spending the nights there give you a chance to meet some of them. I have hiked the last two years and not sure if I will make it or not this year, all depends on the amount of time I can get off from work. It is a good time and suggest getting involved.
Last one to the top buys the first beer!