Whose idea is it for Ralph and Piggy to join the others? What two reasons does jack give for them to join him?

Piggy makes the suggestion, even though Ralph is thinking of it too.  The boys realize that the others have gone to join Jack, even if it is just for the meat.  It is Piggy, however, who offers

"P'raps we ought to go too. Ralph looked at him quickly and Piggy blushed.  I mean--to make sure nothing happens." (Golding)

Jack tells Ralph and Piggy that he will provide them with meat to eat and that his hunters will protect them from the beast. 

Jack makes a convincing argument since he is painted and garlanded like a god and sitting with hunks of meat around him.  His followers do as they are told, or he punishes them.  He demands obedience and gets it.  So his suggestion that his tribe can protect Piggy and Ralph has a ring of truth to it.