Jack tells his tribe that they will hunt, and give a feast and invite the others who have not joined his tribe, to come eat and join them
During the feast on the beach Jack actually gives three reason why the other boys should join his tribe, these are, and I quote... "I gave you food, said Jack, "and my hunters will protect you from the beast. Who will join my tribe?" He then mentions the third reason... "Who'll join my tribe and have fun?"
Simon
Sam and Eric join Jack's tribe in Chapter 11 of William Golding's "Lord of the Flies." This is after Jack successfully splits the group and establishes his own tribe separate from Ralph's leadership.
Simon, Piggy, and Ralph.
The hunters torture Sam and Eric until they agree to join Jack's tribe.
Jack offers to let Ralph's group join his tribe, promising protection, fun, and freedom if they choose to join him. He also suggests that they can have a feast and a party at his camp.
Jack bribe the boys on the island with the promise of meat from the hunting expeditions he leads. He uses the allure of food and power to sway the boys to join his tribe over following Ralph's leadership.
Ralph and Piggy hadn't joined Jack's tribe and when they wondered where everyone was they specifically mentioned Samneric and Bill by name. So, essentially apart from those 5 biguns, Simon and probably the littluns, by the start of chapter 9 most of the boys had already defected to Jack's tribe.
Samneric were tortured and threatened by Jack and his tribe. They were also influenced by the fear and intimidation that Jack instilled in them, leading them to eventually join his tribe to avoid further harm and to align themselves with the more powerful group.
The people on the island with jack AFTER he forms a new tribe, is everyone except for some little ones, Ralph, Sam And Eric ( Though they get kidnapped and forced to join later ) and Piggy. ( Though he gets killed by roger, who had joined the tribe with Jack )
Most of the boys agree to join Jack's tribe, despite Ralph pointing out that he is the chief and that Jack has no shelters to use during the coming storm. Then the storm which has been building up finally breaks and Jack tells the boys to, "Do our dance! Come one! Dance!"