Collective intelligence as characterized by Tom Atlee, Douglas Engelbart, Cliff Joslyn, Ron Dembo, and other theorists, is that which overcomes "groupthink" and individual cognitive bias in order to allow a relatively large number of people to cooperate in one process - leading to reliable action. In this context, it refers to a very rigorous consensus decision making, and may properly be considered a subfield of sociology.
A less anthropomorphic conception is that a large number of cooperating entities can cooperate so closely as to become indistinguishable from a single organism with a single focus of attention and threshold of action. These ideas are more closely explored in Society of Mind theory and sociobiology, as well as in biology proper.
Although most current theorists in the field tend to have a background in artificial intelligence, the 'C.I.' field is actually more reasonably classified as a branch of political science - since it shares the same objective of effective group activity - and rhetoric in common with the philosophy of action, e.g. consensus process. Therefore it is incomplete to characterize the field in terms of theorists with academic credentials - although this is the easiest place to start:
One measure sometimes applied, especially by more artificial intelligence focused theorists like Douglas Engelbart, is "collective intelligence quotient", which presumably can be measured like the "individual" intelligence quotient (IQ) - thus making it possible to determine the marginal extra intelligence added by each new individual participating in the collective.
Critics of artificial intelligence on ethical grounds are likely to promote collective wisdom-building methods, e.g. the new tribalists, the Gaians. Whether these can be said to be collective intelligence systems is an open question. Some, e.g. Bill Joy, simply wish to avoid any form of autonomous artificial intelligence and seem willing to work on rigorous collective intelligence in order to remove any possible niche for AI.
Growth of the Internet and mobile telecom has also highlighted "swarming" or "rendezvous" technologies that enable meetings or even dates on demand. The full impact of such technology on collective intelligence and political effort has yet to be felt, but the anti-globalization movement relies heavily on email, cell phones, pagers, SMS, and other means of organizing before, during, and after events. One theorist involved in both political and theoretical activity, Tom Atlee, codifies connections between these events and the political imperatives that drive them.
See also Smart mob.