Mobile computer technologies are increasingly being appropriated and used to facilitate people’s social life outside the work domain. Addressing this emerging domain of use, we present the design of a context-aware mobile information system prototype facilitating sociality in public places: Just-for-Us. The design of the prototype system was informed by two empirical studies: an architectural analysis of a recently built public space in Melbourne, Australia and a field study of small groups socialising there. We describe these two studies and illustrate how findings informed our prototype design. Finally, we outline an ongoing field study of the use of the Just-for-Us prototype. Categories and Subject Descriptors H5.2. [Information interfaces and presentation (e.g., HCI)]: User Interfaces - User-centered design, Graphical user interfaces, Screen design. General Terms: Design, Human Factors.