To compare influence of each coalition is an important topic in analysis of group decisions because it leads to appropriate analyses and estimations about the final decision and the coalition formation. In this paper, group decisions are mathematically modeled as simple games in the framework of the theory of cooperative games. A notion of coalition power index (CPI), which is extended from the concept of power index to enable evaluation of coalition influence, is employed for the comparison. This paper takes an example of group decision situations in reality and provides a specific calculation result of coalition influence, using the blockability index among other CPIs. Then, the result is compared to results of using other important power indices such as the Shapley-Shubik index.