A group of porpoises is referred to as a pod. Porpoises tend to travel in very small groups, according to Diffen.com, which analyzes some of the differences between dolphins and porpoises.
Porpoises are very shy and usually have pods that consist of just a few individuals. In contrast, dolphins congregate in larger groups and are much more social. Porpoises sometimes spend time alone rather than with other porpoises. According to Diffen, porpoises are not very suitable to being used in animal shows at aquariums, because they are less social creatures than other animals. They also have more fear of humans than dolphins do.