He takes his group to the platform because Piggy wants to get his glasses back from Jack and the other hunters. And Ralph wants to get his authority back.
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Ralph was standing on the platform blowing the conch when Johnny arrived. Piggy helped Johnny to climb onto the platform, where he and squatted down on the ground in front of Ralph.
At the begining of the chapter, while Ralph and Piggy are bathing in the pool Ralph asked "Where's everybody?" Ralph specifically refers to Samneric and Bill, which would indicate that he believed that there were at least five boys in his group, including himself. Piggy pointed beyond the platform and replied "That's where they have gone. Jack's party." So in fact all that actually remained of Ralph's group were Ralph and Piggy. By the morning after the party, and the killing of Simon, Samneric had rejoined Ralph's group.
Because 1: Piggy and Ralph got the conch they used as a talking power in the assemblies; 2:Ralph is still the leader after all; 3: Piggy owns the specs, he has the power to take what he owns back in a civilized group(but apparently it is not)
Ralph is able to persuade the other boys that they will be saved by sharing with them that his father is a navy captain. He tells them that the queen has a room with maps that show every island in the world.
The two reasons of why jacks group is more exciting than Ralph is because jacks tribe hunt and have a feast and have fun.