Group decision-making is a participatory process in which multiple individuals collectively perform situation analysis, think of alternative courses of action, and select the best alternative to solve the problem. Similarly, in the context of Self-help groups, group decision-making is a necessary process. However, decisions made collectively tend to be more effective than decisions made by a single individual. Still, group members face various constraints while group decision making like social pressure toward conformity, individual domination, conflicting secondary goals, undesirable compromises, ambiguous responsibility, and time. Most studies suggest that SHG members perceived problems faced during group decision-making as a significant constraint. Considering a shortage of empirical research indicating the exact reason for group failures, what makes a group successful, and the factors that lead to ineffective group decision-making, the study aimed at systematically and scientifically developing a group decision-making index to study the group decision-making of the Self-help groups, various factors affecting the group decision-making process, and quantitatively measuring how different groups vary in their group decision-making ability, Thus, an instrument was developed using a two-step method, i.e., instrument designing and judgmental evidence. After that, the validity of the instrument was computed through Item-Content Validity Index (I-CVI) method. Finally, it was narrowed down to 48 statements distributed among ten indicators. The reliability coefficient of the tool was found to be 0.80. Thus, it was found that the group decision-making indexes had appropriate content validity and internal consistency to measure and quantify the group decision-making process of selected Self-help Groups. The study recommends the use of the developed index for studying the root cause of ineffectiveness in group decision-making in various Self-help groups, which will help in the formulation of strategy for overcoming the constraints related to group decision-making as reported by various SHGs.