- We examine how group membership influences individual decisions with respect to joining a mass political struggle, under the assumption that group members have a strong group identity, expressed by a strong commitment to group decisions. We suggest a two-level theoretical game model in which, in the first stage, an individual calculates the costs and benefits of participation at the group level and then he/she calculates the costs and benefits of the group's participation in mass collective action. The model shows that when the costs of action are low and the expected benefits are high, there are two equilibria - one with high and the other with low probability of collective action. It also shows that the chances of achieving political change through mass mobilization are lower when individuals are members of two subgroups that act separately, than when they are members of one group only. The model is applied to the socio-political processes in Poland between 1976 and 1981.