Drawing on Tajfel and Turner’s Social Identity Theory and Harris’ Group Socialization Theory, the current study examined how group processes (i.e., group membership and social status) contribute to schoolchildren’s bystander reactions to hypothetical bullying. A between-groups experimental design was used to examine the effects of group membership (e.g., belonging to the same group as the bully, victim, both characters, or neither) as well as bully social status (e.g., more or less popular than the bystander) on the emotional and behavioural reactions of 357 middle-school students in grades 6 to 8. Identification with the victim was associated with greater likelihood of bystanders endorsing feelings of anger. However, witnesses who observed an in-group bully harassing an out-group victim reported the strongest feelings of shame. Feelings of shame and anger subsequently predicted bystanders’ willingness to help the victim, whereas feelings of sadness and fear positively predicted intentions to talk to an adult. Results are discussed in light of the small but growing body of literature on the intersection of group processes, moral emotions and bystander behaviour. Implications and recommendations for school-based anti-bullying interventions are provided.