Why did Ralph and Piggy join the feast in Lord of the Flies?

Jack dismisses Piggy with, "because we don't want you," essentialy because he doesn't like him and doesn't want his company. Ralph sugars the dismisal by telling Piggy that he has an import job to do at the platform (to take names). This helps to show that while Jack's decisions are mainly made in his own self interest Ralph does show some compassion and consideration towards other people. Piggy, in spite of the fact that the expedition is likely to involve physical exhertion, wants to accompany Ralph and Jack to establish himself as one of the "important" decision makers among the boys.