Throughout the novel Lord of the Flies, Jack is in charge of a group of boys. The members of that group change as the story progresses. At the beginning of the novel, Jack is the leader of the choir boys, and as the collection of boys tries to make plans for survival, the choir boys become the hunters, who are still led by Jack. Being in charge of this smaller group of hunters is not enough for Jack, and so he begins to enact a plan that will make him Chief.
When Jack splits his group from the rest of Ralph's boys, he adds more members, and some of the boys step into larger character roles. Roger becomes a second-in-command and enforces Jack's requests amongst the group through use of force. The littluns also join Jack's camp, but by this point in the novel their allegiance is not very important, because they seem to have been almost forgotten by the older boys on the island. The last boys to become part of Jack's group are the twins, Samneric. It could easily be argued that they do not wish to be a part of Jack's group and are forced to participate on his behalf. As they tell Ralph on page 146, "—they made us. They hurt us—."
Simon, Piggy, and Ralph are the only boys on the island who do not become members of Jack's group. By the conclusion of the novel, all of the boys left on the island have aligned themselves with the "savage" tribe, and with Piggy and Simon gone Ralph is left to represent the "civilized" tribe on his own.