A Compsognathus is the smallest dinosaur ever discovered, I believe. It's a theropod dinosaur, meaning it was a predator (although some theropods were herbivores, belonging to the therizinosaur family). Like T-Rex, Compsognathus had only two fingers on its hands. It was likely to be an insectivore, I believe, perhaps also preying on small lizards and other small animals.
Despite what's depicted in The Lost World, there is no evidence to suggest Compsognathus was capable of killing prey the size of a human, and I don't think there is any evidence to prove they were pack animals either. As far as I know, Compsognathus would have been relatively harmless to a human.