Participation in collaborative advocacy organizations is one of the most common advocacy tactics pursued by nonprofit organizations. However, field-level dynamics and norms around collaboration may be changing with the growth of public-private intermediary organizations. Using a lens that brings together theories of structuration and institutional entrepreneurship, this research investigates (a) how intermediary organizations structure advocacy opportunities and institutionalize new advocacy practices at the field level, and (b) how member organizations interpret those opportunities and practices. Qualitative findings from a regional homeless services policy field demonstrate that intermediary organizations gain members and thus, power, based on their position in the policy field and through their ability to connect members to valuable government contacts. In this field, participation in public-private intermediary organizations has surpassed involvement in traditional advocacy coalitions as providers are motivated to meet organizational legitimacy goals as much as advocacy goals.