Prototyping is widely recommended as an excellent mechanism for requirements elicitation and validation. However, few details are available on how prototypes should be used for this purpose, especially in a group environment. The goal of this research is to develop and evaluate a methodology for using prototyping to elicit and validate requirements from large user groups. An initial prototype evaluation methodology was developed, assessed during a pilot case study, and revised to support different evaluation phases and types. The revised participative prototype evaluation methodology provides a specific structure for each prototype evaluation phase with detailed methods and GroupSystems tools defined for each evaluation type. An overview of the face-to-face procedures by evaluation type is included in this paper.