It's great!!!! Our junior troop went last year for the 1st weekend of this.
Let's see, I think I'll arrange this as: where to stay, what you do, things you might not know or think of.
Where to stay:
Girl Scouts usually offer 3 options: camping, dorm living, or hotels with staggered prices (obviously cheapest to most expensive listed above). The camping is at several local campgrounds, and is usually closest to the park. We electd the dorm living, and were housed at Elizabethtown College, which was probably 20 minutes away from the park. We passed two of the campgrounds on our way to the park, but it was an easy drive. I'm not sure where the hotels were, cause they were pricey. All the options include some food (usually breakfast Saturday, dinner in the park Saturday night, and breakfast Sunday). For the campgrounds, they bring in packaged foods like bagels and danish's. There are comfort stations, but you're camping in tents. The weekend we went, it absolutely POURED! I was never so happy that we spent the $15 more per person to stay in the dorm!! We went to the campground Sunday morning for the swaps, and everything was just soaked! In the dorms, your food is from the college cafeteria, so you get hot breakfasts. You also had access to the indoor pool for scheduled swims. I think our price was $90 per person, including the park admission.
What you do:
Obviously, you visit the amusement park

but they also have a singalong scheduled for Saturday evening at the ampitheater, and there is a swap even set up for Sunday morning at the main campground.
Things you might not know:
With Hershey Park tickets, you can do a "sneak peek" the day BEFORE your tickets. It's usually for 2-3 hours just before park closing. This gives you a good chance to get a feel for the park layout, and schedule meeting times and locations, so you're all set when the actual day event starts.
You will be charged for parking. One of the attendants let us in when we explained we were with the GS event, but the parking isn't really included. I think it's something like $6 a day.
You can visit chocolate world for free. It's mainly a big gift store with a "tour" through a simulated factory. That's where they give you your free chocolate (a snack sized hershey bar). They also have limited edition chocolates at Chocolate World that aren't available other places. It's a good idea to visit this on the Friday of the preview, before you go into the park, because it's a madhouse after the park closes, and if you leave chocolate in your car all day Saturday it might melt.
Have a great time, and ask if you have any other questions!