Despite the importance of trustees as leaders in US higher education institutions, our knowledge and understanding of their behavior is limited. This is increasingly problematic as trustees engage more directly with institutions as institutional boundaries have become more porous. We utilize social network analysis and document analysis of exchanges to explore trustees' involvement in a qualitative comparative case study of four elite US research universities. We draw on the microfoundations tradition of neo-institutional theory to frame and evaluate how the actions of these individuals reproduce, expand and reorganize these institutions and their boundaries. Results show that these leaders are heavily involved with the universities they govern, but in widely varied ways and to different degrees. We inductively derive two forms of trusteeship -- traditional trusteeship (e.g., governance) and expanded trusteeship (e.g., capacity building and collaborative partnerships) -- that occur unevenly across our four institutions. These findings demonstrate that the nature of trusteeship at US research universities varies across institutions in profound ways that have substantial consequences for their boundaries, behaviors, and governance as well as the organizational stratification in the field of US higher education.