In software engineering education, team composition in project courses represents a persistent challenge. This study investigates how interpersonal connections influenced teamwork quality (TWQ), team success, and team performance in a software engineering capstone course, involving 54 student teams. Using a mixed-methods approach, we analyzed 468 survey responses and conducted 19 semi-structured interviews. Teams were categorized based on interpersonal familiarity: predominantly composed of members with two prior connections, primarily formed from members with one existing relationship, mixed configurations (with both familiar and unfamiliar members), and primarily unacquainted students. Teams with established relationships demonstrated significantly higher TWQ scores and team success, benefiting from enhanced trust and cohesion. However, these teams also encountered challenges with subgroup formation and imbalanced workload distribution. Mixed teams achieved the highest balance of contribution scores, indicating more equitable task allocation. Conversely, teams of mostly unacquainted members reported the lowest TWQ, team success, and team performance, struggling with coordination and collaboration. These findings highlight the complexities of team composition in software engineering education and provide insights into key considerations for assembling student teams.