This paper explores the link between membership size and the governance of grassroots associations. The issue of mission preference heterogeneity is highlighted and its effect on membership size is analyzed by developing a model and conducting a numerical simulation. An important finding is that the degree of heterogeneity of mission preferences in the potential member population has a negative effect on the optimal number of members of the grassroots association. The paper ends with a discussion of the use of governance mechanisms to limit mission drift.