Why do Ralph and Piggy join the dance in Lord of the Flies?

Plain and simple: Peer Pressure.  They needed to be part of this social activity which is easy to get caught up in with the rhythmic "beating" of their voices like drums as they sing and dance around the fire.  Remember, they have been on the outside in their small group for quite some time, and it is a wonderful feeling to be part of something larger--to belong and not be targeted.  There is safety in numbers.  It is also very primitive in bonding and preparing for the hunt or celebrating the victory of a hunt, which is the case here.  They were reenacting the hunt for the boar, celebrating their success, cooking the meat, and eating.  Maybe moreso than anything else, Ralph and Piggy were hungry for good, hot, nourishing protein.