They were kidnapped shortly before Piggy's death when they went with Ralph and Piggy to Castle Rock. They were tortured by Roger into swearing allegiance to Jack.
jacks tribe on ch. 11
The hunters torture Sam and Eric until they agree to join Jack's tribe.
At the end of chapter eleven samneric were taken by Jack and his tribe and forced to join Jack's tribe. In twelve they have to work with the tribe to try and find Ralph on the island. Nothing real serious happens to them. they don't get hurt of killed.
Samneric choose to join Ralph and Piggy in Chapter 12 of "Lord of the Flies," after they are captured by Jack's tribe and forced to betray Ralph's hiding place. They express regret for their actions and seek safety with Ralph's group.
Samneric because they where forced to join Jack Mildrew's tribe and they told Ralph about Jack's plan to search and destroy Ralph.
Ralph learns of Jack's plans for him through Samneric, who are forced to join Jack's tribe and spy on Ralph's group. Samneric bring Ralph news of Jack's intentions to hunt him down and kill him. This betrayal shows Ralph the extent of Jack's animosity towards him.
Initially there is only one group of boys but right from the start Jack is a dissenting voice. Eventually Jack leaves and forms his own separate tribe. Near the end of the book all the boys with the exception of Ralph, Piggy and Samneric have joined Jack's tribe. Piggy is then killed, Samneric are captured and forced to join the tribe, leaving Ralph as the only boy who isn't a member of the tribe.
During the confrontation on the causeway leading to castle rock Jack ordered his tribe to grab Samneric and tie them up. Later Sameric told Ralph that they were made to join the tribe and that Jack and Roger had, "hurt us--"
Samneric are twin boys on the island in "Lord of the Flies." They start off loyal to Ralph, but eventually succumb to Jack's influence and join his tribe. Their loyalty to each other is a central theme in the novel.
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.
You need to think about why its important and maybe that will help you find the answer rereading may be an option too
No one will accept you if you are weak. But if you do want to join as tribe, then ask the that tribe leader and he invites you, then your in!