For indoor visible light communication (VLC) systems, multiuser spatial multiplexing (SMP) has become a crucial technology to enhance the network capacity by using multiple light-emitting diodes (LEDs). However, the performance of a VLC system based on multiuser SMP is dynamic-range-limited since the dynamic range of some LEDs is usually underutilized in this case. To solve this problem, an opportunistic non-orthogonal multiple access (NOMA) is put forward to utilize the unsaturated dynamic range by allowing more user connections and as a result the spectral-efficiency is improved. Therein, the effects of user location distribution and successive interference cancellation (SIC) error propagation are investigated. In order to further improve the spectral-efficiency, we have developed an adaptive dynamic-range utilization scheme. Simulation results demonstrate that the proposed scheme effectively improves the spectral-efficiency under the primary users' quality of service (QoS) requirement.