This paper explores the multifaceted role of group composition and the influence of newcomers on group performance and dynamics. Group composition - including diversity in skills, traits, and backgrounds - significantly affects group processes such as cohesion, communication, and decision-making. While stability fosters trust and shared memory systems, controlled turnover can inject valuable perspectives and increase adaptability. Newcomers exert both unconscious and conscious influence. Unconsciously, their arrival can reshape group norms even before formal integration, while conscious influence depends on their motivation and the group's openness to change. Newcomers can positively stimulate creativity and group reflection, yet they may also disrupt harmony and demand increased resources for integration. The outcome of their influence is largely shaped by how well existing members manage receptivity and adapt to change. Thus, strategically managing group composition and supporting newcomer integration is essential to optimizing team functionality.