Astrobees are free-flying autonomous robots becoming useful in offloading repetitive and simple routines necessary for the monitoring and maintenance of the International Space Station (ISS). They are the cornerstone of automated space explorations as the capability of working as a group becomes a critical role. In this paper, we propose a novel event-trigger-based decentralized nonlinear model predictive control framework accommodating dynamic formation control in a leaderfollower fashion. The proposed framework generates collision-free trajectories using the approaches for exclusive region obstacle avoidance and safe maneuvering around constrained environments. The simulation results demonstrate the efficacy of the developed framework with two different scenarios from one module to another in ISS through a constraint entrance.