In this paper we present a floor-based user interface (UI) that allows multiple users to explore a spatial augmented reality (SAR) environment with both monoscopic and stereoscopic projections. Such environments are characterized by a low level of user instrumentation and the capability of providing a shared interaction space for multiple users. However, projector-based systems using stereoscopic display are usually single-user setups, since they can provide the correct perspective for only one tracked person. To address this problem, we developed a set of guidance cues, which are projected onto the floor in order to assist multiple users regarding (i) the interaction with the SAR system, (ii) the identification of regions of interest and ideal viewpoints, and (iii) the collaboration with each other. In a user study with 40 participants all cues were evaluated and a set of feedback elements, which are essential to guarantee an intuitive self-explaining interaction, was identified. The results of the study also indicate that the developed UI guides users to more favorable viewpoints and therefore is able to improve the experience in a multi-user SAR environment.