<p>Organizational members are essential for nonprofit trade associations (TAs) to achieve their goals, but the literature provides limited exploration of barriers to joining TAs. This paper investigates such barriers from the point of view of managers of TAs in the American charter school sector. The analysis reveals multiple explanations for why schools may not join their associations, and four explanations are discussed in depth: policy disagreement, free riding, low nonprofit resources, and high nonprofit resources. Considering the mechanisms that underlie these explanations leads to the suggestion that three factors —free riding, policy disagreement, and high resources —act as barriers to engagement.</p>