We developed Tourgether, an app that enables tourists' mutual sharing of their experiences via check-ins in real time, to enhance their awareness and exploration of various points of interest (POIs) in a tourism region. We conducted formative studies and a between-subjects field experiment to assess how tourists used Tourgether in their travels, and the influence of real-time experience-sharing on unplanned POI visits, respectively. The results of the formative studies indicated that seeing shared real-time experiences encouraged tourists to explore and make unplanned visits to less well-known POIs, and that their decisions to make unplanned POI visits were dependent on familiarity, worthiness, and convenience. The app also created a feeling of co-presence among tourists, boosting their desire to interact with others. Two strong motivators for tourists to check in on the app were identified: contributing to other tourists, and recording journeys. Our experimental results further showed that seeing shared real-time experiences prompted the participants to make more unplanned visits than would have been the case if they had not seen them. This influence was more prominent among tourists who planned more POI visits. Other differences in the usage and influence of Tourgether across these two groups will also be discussed.