||Group Decision Support Systems (GDSSs) are computer-based systems which assist the solution of ill-structured problems by a set of decision makers. In the past few years, there is an increasing interest on developing GDSSs for different domains. In order to present real life problems in a structured manner, many systems adopt a logical view in which arguments are linked in terms of their relationships. Hermes is a GDSS based on such a logical model, which can be used for asynchronous collaboration of geographically distributed decision makers through a WWW browser. Unlike previous systems, Hermes provides reasoning mechanisms which guide the group decision making process by suggesting the best solution based on the current status of argumentation. In this work, we propose and implement an enhanced argumentation framework for Hermes. The framework utilizes Multiple Attribute Decision Making (MADM) techniques which aim at finding the most rational choice among a set of potential alternatives. The new framework significantly extends the original design and increases the applicability of Hermes in decision making environments where alternatives have quantifiable attributes.