Why camp with a group?
Each person who pre-registers for Pennsic is allotted a certain number square feet of camping space (usually 250 sq ft/person). When people pre-register as part of a group, their individual land allotments are pooled together and the group then gets one large chunk of land, which they can subdivide and arrange as they choose.
By camping as groups, people are able to pool their resources. Instead of each person bringing their own camp kitchen, the group might instead arrange to bring one large one that will feed the whole group on a meal plan. Groups of families might make arrangements where the adults take turns watching all the children, allowing others to have free time. Some groups arrange their tents so that there are common areas for socializing and parties. Others focus on creating period and/or themed encampments. Aside from these considerations though, there is another very good reason for camping with a group if you're new to Pennsic - you'll be surrounded by people who are more experienced. Then, if you discover you've forgotten something or need help in some way, chances are good that one of your campmates will be able to assist you.
There are many different types of groups. Some are composed of residents of a Barony, or members of a household. You'll also find guilds or people with similar interests of study, along with all sorts of other collections of people. While some groups are very restrictive as to whom they'll allow to camp with them, many are very open. Try asking around in your local area or among people you know to see if they know of a group that you can join.