For group decision about shared goods, the nature of the shared good and how its cost is to be shared among group members must be determined. Complexity arises from heterogeneity in preferences and endowments and nonlinear cost. To facilitate group decision, this paper proposes special type of group decision support system, a cost share adjustment process (CSAP), in which cost shares are adjusted iteratively via algorithmic rules until agreement is reached, ideally producing a socially optimal, cost feasible, and fair outcome. Design elements for CSAPs include message space, cost allocation and adjustment rules, controllers, and incentive rules, with many possibilities for a cost share adjustment process. In contrast to public good literature, our designs apply for situations of nonlinear cost, with economies of scale and fixed costs. Because of impossibility theorems, a design approach is developed: simulation and economic experiment are employed to compare alternative designs. As simulation and experiment both indicate, complicated rules for incentive purposes may impede locating group agreement. Instead of complicated economic incentive rules, economic experiments show that unanimity Approval Voting can mitigate the effects of strategic behavior.