How do communitarians disagree with social contract theory?
By focusing on the individual as part of a community, Communitarianism seeks to equalize rights and responsibilities across all members instead of delineating social classes.
Social Contract Theory states that individuals will agree to social rules and regulations based on their mutual desire for protection and safety, rather than by coercion.
Communitarians usually disagree with social contract theory because of the implicit nature of the social contract itself; since they feel that a community should consist of like-minded people who want the same good outcomes across the board, they are put off by the idea that people will only accede to the binding membership for their own individual protection, rather than for the good of all. Communitarianism is, at its core, a type of collectivism, although it usually seeks to emphasize individual importance in the community; meanwhile, social contract theory places great emphasis on the individual's right to support or oppose the mutually-agreed-upon society, but tosses out anyone who refuses to be a member. Communitarians generally agree that the value of the individual is both shaped by and dedicated to the community, and so discard the idea that membership is voluntary; members are born and die as members.
Today, communitarianism is a low-level philosophy, with few proponents, while social contract theory is practiced in all governmental societies but rarely acknowledged as such.