The development of emotional competence is an important milestone during early childhood. Beyond early experience within the family, the (preschool) classroom is a relevant socialisation context, and both teachers and peers may contribute to children's emotion-related outcomes. Tracking changes in the emotion regulation competence of N = 173 preschool children (age in months: M = 43.3, SD = 6.6, 45% girls) over 6 months, the current study investigated whether and how peers contribute to differences in the development of emotion regulation in preschool classrooms. The research questions were addressed by examining three different kinds of peer groups: Classroom-level peer group, friendship group, and high-status peers (social network centrality). Teachers rated children's emotion regulation competence and empathic co-regulation behaviour--referring to preschoolers' supportive responses to their peers' emotional expressions--at three assessment points. Multilevel growth modelling provided evidence that friendship-group empathic co-regulation (empathy, helping, and comforting behaviour) predicted changes in preschoolers' emotion regulation over time, confirming that friendship groups influence young children's emotional development.