Joint Spatial Division and Multiplexing (JSDM) is a massive MIMO (mMIMO) scheme in which the beamforming is split into two stages, decreasing the channel estimation overhead and mitigating the interference. Moreover, Reconfigurable Intelligent Surfaces (RIS) have emerged as a hot research topic in recent years, being widely advocated as a candidate technology for next-generation wireless communications. These surfaces passively alter the behavior of propagation environments ameliorating the performance of wireless communication systems. However, this gives rise to even larger channel dimensions than conventional mMIMO systems, causing a surge in the estimation overhead. In this paper, we investigate the downlink of a single-cell Multi-User MIMO (MU-MIMO) system employing JSDM with the aid of a RIS installed between the base station (BS) and the users having a weak direct BS link. The RIS provides an alternative communication link, and JSDM reduces the channel estimation overhead. We briefly explain the concepts of JSDM and spatial signatures. Then, we introduce a novel similarity measure suitable for clustering the users in the presence of a RIS. Furthermore, we propose to add a precoding stage to the overall JSDM precoder to remove undesired effects induced by the use of a RIS in the considered MU-MIMO scheme. Simulation results show the improved performance of RIS-aided JSDM in comparison to the conventional scheme when the RIS is large enough for a given transmit power with a gain in the average sum rate, reduced effective channel dimensions and increased cell coverage.