The bulk of the produced electricity powers buildings; 120 million homes and 5 million commercial buildings dominate the U.S. energy consumption. Creative design of Buildings-to-Grid (BtG) integration methods is an essential component of smart cities. Buildings' occupancy behavior is a crucial component of successful BtG integration, as buildings become more sophisticated and people spend more time in offices and cities-making occupancy behavior one of the leading factors in energy consumption and thus largely impacting power grid dynamics. In this paper, we study the impact of integrating occupancy-based building dynamics and constraints with power grid transients, while focusing on frequency regulation. First, dynamics of building clusters and building-integrated power networks are presented-both operating at different time-scales. Second, occupancy-based building constraints are discussed. Third, the time-scale discrepancies are investigated, and a model predictive control-based algorithm that formulates occupancy-based BtG integration is given. Finally, case studies demonstrate the impact of the proposed framework on energy savings and significant frequency regulation.