Presence of others. Historically, this was one of the first topics studied in social psychology, specifically with work on crowding, affiliation (Schachter), social comparisons (Festinger), social facilitation (Zajonc). With Asch's influence, attention turned from the influential presence of other people to the individual's subjective perceptions of social stimuli.

Conformity, Obedience. Three classics: Sherif's studies on norm-formation in response to the "autokinetic effect"; Asch's studies on conformity and the reinterperetation by Ross, Bierbrauer, & Hoffman (1976) (see here [first part] for further information); and Milgram's obedience experiment (see here for the various follow-up experiments). A more recent classic are the bystander intervention studies by Darley and Latane, which also led to one of the best phenomenon-specific theories in social psychology.

Persuasion (= intentional influence of somebody else's attitude or behavior). Two major historic lines: Attitude change research by the Yale group around Hovland (1950s), and behavior change ("compliance") research inspired by dissonance theory and its attributional interpretations. Cialdini's "six weapons of influence" nicely summarize several persuasion techniques. (See here [second part] for further information.)

Resistance. Reactance to and awareness of being influenced are two factors that can inhibit persuasion success. There is also some research on individual difference factors that seem to "inoculate" people somewhat against social influence -- high self-esteem, low social anxiety, need for individuation, "masculine" gender role (regardless of actual sex!).