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Interfacing cold atoms with integrated nanophotonic devices could offer new paradigms for engineering atom-light interactions and provide a potentially scalable route for quantum sensing, metrology, and quantum information processing. However, it remains a challenging task to efficiently trap a large ensemble of cold atoms on an integrated nanophotonic circuit. Here, we demonstrate the first direct loading of an ensemble of nearly a hundred atoms into an optical microtrap on a nanophotonic microring circuit, with a trap lifetime approaching one second. Efficient trap loading is achieved by employing degenerate Raman-sideband cooling with a built-in spin-motion coupling in the microtrap and a single optical beam sent from free space for optical pumping. We show that the trapped atoms display large cooperative coupling and superradiant decay into a whispering-gallery mode of the microring resonator, holding promise for explorations of new collective effects. Our technique can be extended to trapping a large ensemble of cold atoms on nanophotonic circuits for various quantum applications.