Many tools help GUI designers to produce prototypes of their future applications, but when this job requires the collaboration with geographically dispersed partners some problems arise like lack awareness of the others' work, concurrent manipulation of virtual objects and non-fluent communication of actions and intentions. We Sketch allows a small geographically distributed group to build GUI prototypes in a highly participative manner, keeping everyone aware of what is going on. A 3D virtual round table, virtual sheets simulating sketches, avatars representing users, an audio communication system and a shared view of sketches are the pieces that allow designers to discuss and co-design GUIs in real time offering a comfortable and pleasant experience. This paper describes the tool and the results of a set of experiments conducted in order to validate the hypothesis: "WeSketch allows a small group of designers who are geographically dispersed to be more efficient and feel more motivated during the GUI Sketching task than they do with current tools".