Many employees in today’s organizations are concurrently involved in more than one team at the same time. This study investigates the individual-level relationships between such multiple team memberships (MTM) and employees’ wellbeing and job performance. Building on the challenge-hindrance stressor framework and research on socialization processes in organizations, we hypothesize that employees may appraise their MTMs as a challenge or a hindrance, depending on their work experience. We examined our predictions with multi-source data from a large organization in applied research (N = 1211). Results confirmed that less experienced multi-teamers were less likely to appraise MTM as a challenge and more likely as a hindrance, resulting in higher absenteeism and lower performance levels. MTM was unrelated to absenteeism and performance for more experienced employees. These findings provide important insights into the mechanisms and contingency factors that shape the wide-ranging consequences of MTM for individual employees.