This article examines effects of the social group on individual alcohol and drug use upon entry and exit from the club. Based on collected biological measurements of alcohol and other drug use, this study explores whether social group indicators (e.g., group characteristics) are predictive of alcohol and other drug use for individual club patrons. A total of 368 social groups, representing 986 individuals (50.7% female), were anonymously surveyed, and biological measures of alcohol and other drug use were collected at entrance and exit to clubs on a single evening. Both individual and group-level indicators were assessed. Because data were clustered by club, event, and group, mixed-model regressions were conducted to account for non-independence. Group indicators of high blood alcohol concentration were being in a group that intends to get drunk, that has at least one member who regularly gets drunk, and that has discrepancies in its expectations regarding drug use. Group indicators related to cocaine use were high levels of drug use expected among group members, little discrepancy among the group members regarding the drug use expected, and high levels of intentions to get drunk. In addition, older groups were more likely to have higher levels of cocaine use. There were less consistent findings regarding group effects on marijuana use. The most consistent finding was that high drug use expectations were related to higher levels of marijuana use. Together, these data suggest that strategies should focus on recognizing group indicators as risks for group members. Promoting social responsibility for group members may create safer club experiences among young adults. These efforts could model designated-driver programs as a way to increase safety and social responsibility.