Opportunists are people who see a chance to gain some advantage from a situation, often at the expense of ethics or morals. An opportunist seizes every opportunity to improve things for himself.
Say you won millions in the lottery. People would come out of the woodwork hoping to get their hands on some of it. These people act as if they are close friends. But they are not; they are opportunists. Famous opportunists include “carpetbaggers,” Northern opportunists who, after the American Civil War, poured into the South to turn Reconstruction into personal financial gains.