This paper studies how exclusive social groups shape upward mobility, and whether interactions between low- and high-status peers can integrate the top rungs of the economic and social ladder. Our setting is Harvard in the 1920s and 1930s, where new groups of students arriving on campus encountered a social system centered on exclusive old boys' clubs. We combine archival and Census records of students' college lives and long-run careers with a room-randomization design based on a scaled residential integration policy. We first show that high-status students from prestigious private high schools perform worse academically than other students, but are much more likely to join exclusive campus clubs. The club membership premium is large: members earn 32% more than other students, and are more likely to work in finance and join country clubs, both characteristic of the era's elite. The membership premium persists after conditioning on high school, legacy status, and even family. Random assignment to high-status peers raises the rates at which students join exclusive social groups on campus, but overall effects are driven entirely by large gains for private school students. In the long run, a shift from the 25th percentile of residential peer group status to the 75th percentile raises the rate at which private school students work in finance by 41% and their membership in adult social clubs by 26%. We conclude that social interactions among the educational elite mediate access to top positions in the economy and society, but may not provide a path to these positions for underrepresented groups. Differences in academic and career outcomes by high school type persist through at least the class of 1990, suggesting that this causal channel remains relevant at contemporary elite universities.