An agile software development team relies on communication and collaboration to perform requirements engineering activities, rather than on dedicated analysis tools or documentation. Evidence from practice indicates that two simple physical artefacts (story cards and the wall), used in a particular and disciplined manner, and supported by appropriate social activity, are key to the success of co-located agile teams. However, little is known about this social activity or how communication and collaboration supports requirements activities in this setting. This paper reports an empirical study of a commercial agile team to investigate this issue. Using a combination of qualitative data collection and cognitive analysis techniques, we found evidence of gathering, evolving and clarifying requirements that are managed through patterns of communication. These patterns suggest that a form of situated conceptualization, which we have termed 'shared conceptualization', underpins the team's requirements engineering activities.