Co-offenders commit more violent offenses than solo offenders, in part due to collective behavior processes that exacerbate behavior in groups. But, different co-offenders likely experience these processes differently. Following this, this research examines the relationship between co-offending, group sex and age composition, and violence using National Incident–Based Reporting System (NIBRS) data. Results indicate that male groups are more likely to seriously injure victims, but that sex composition matters most in small groups; in larger groups, serious injury is likely regardless of sex composition. Furthermore, age is related to violence such that the presence of more young adult offenders is associated with increased likelihood of serious injury. Taken together, this research suggests the importance of considering group composition in the situational production of group violence.