Bobby, Skeezie, Joe, and Addie are misfits because they are seen as outsiders in the seventh grade social world.
The four who form "the gang of five" are different than the prevailing social order. Bobby is overweight, while Skeezie looks contrary to accepted middle school conformist ideas. Joe is seen as a "faggot" while Addie is teased because of her intelligence and height. The result of this is that they do not find a niche in their seventh grade world. Students and adults marginalize them because of their creative and intellectual distinctions. Peers like Kevin and DuShawn push them to the periphery while authority figures like Mr. Kiley and Ms. Wyman do not understand them. As a result, the "gang of five" are misfits.
The "gang of five" use their misfit label to their advantage when they run for political office. Their entire premise of the "No-Name Party" is in speaking for fellow misfits. They use their eccentric nature to their own political advantage. Part of the reason their message resonates is because they speak from personal experience. The reaction after Bobby's speech about his own experiences of being different because of what he "puts in a sandwich" reflects this. The group authenticates the experience of being perceived as different, or someone who does not "fit" neatly into the social order.