sr1, obviously we want you to stay for the whole day, and I think everybody here agrees that you will most likely want to stay after the first two sessions. The intro sessions were made available for people just like you though, riders who think they want to get in to track days but would like to test the water first. This is one of the many things that makes NESBA the best club around.
The Intro riders are assigned a Control Rider that works with the Intro group for the first two sessions. These sessions are led by the CR and the pace is controlled so that you can become familiar with the track without going too fast too soon. This is a great opportunity for first time track riders to get some one on one with a much more experienced rider, ask questions, and learn the basics of riding on the track. Intro groups are small, usually ranging from between one and five riders. Obviously Control Riders are there to answer your questions and work on track with you all day long, but the advantage to the Intro group is you have a specific guy assigned to you.
You can always pay and sign up for the rest of the day after the intro sessions are over. We always maintain enough of a buffer in the beginner group that the Intro riders will have the chance to stay for the rest of the day if they want. Since it sounds like you don't have any previous track experience you might want to take advantage of this benefit. We also have a lot of first time track riders who go straight to beginner and do very well, however I think the ones who do the best are very experienced street riders. If I were you I would take into consideration your own street riding experience, and what you percieve your level of comfort will be with the higher speeds on the race track and decide from there.
Keep in mind that if you start in Intro and then sign up for the rest of the day in Beginner, your bike will need to meet the basic technical requirements of the Beginner group before you join the remaining sessions.
Good luck with whatever you decide...I know you'll have fun!