Semi-Grant-Free (SGF) access for Non-Orthogonal Multiple Access (NOMA) has recently emerged as a hybrid scheme that integrates contention-free access for Grant-Based (GB) users with contention-based access for Grant-Free (GF) users. In this context, we consider multiple GB users scheduled via a conventional grant-request procedure to dedicated Resource Blocks (RBs), which are then opportunistically, though transparently, shared by GF users to enhance network capacity while reducing control signaling overhead and service latency. Subsequently, the challenging problem of grant-free multi-RB multi-user scheduling and admission control under strict physical layer security constraints and Quality-of-Service (QoS) requirements is addressed, aiming to maximize the number of admitted GF users and minimize their transmission power. To this end, two scheduling algorithms are designed and proposed. First, a one-shot, low-complexity greedy algorithm prioritizes GF user access to RBs and respective Signal-to-Noise-Ratio (SNR) levels based on their backoff times to the base station. Secondly, an iterative coalition game-based algorithm refines RB and SNR level scheduling decisions, with GF users acting independently as players that aim to further minimize their power consumption. The proposed scheduling framework achieves a superior tradeoff between the number of admitted GF users and power consumption, while satisfying all users’ service and secrecy requirements, outperforming conventional scheduling strategies from the literature.