Group living provides various advantages to individuals such as predator protection, improved competiveness against conspecific groups for resources, and better breeding and exchange of information. However, close spatial proximity commonly comes with costs in the form of increased competition between individuals over limited resources within the group. In the competion, unless a decision is reached among indivudals, the group will split apart and indivuduals will loss advantages in group living. To keep the group cohesive, self-organization and leadership are two means of coordination and two types of decision can be made on the group level: combined decision and consensus decision. Here we briefly review the types of group decisions, the coordination mechanisms of competition within group and the process of group decision making. We also compare the effect of information and interest in group decision making. We tend to provide backgrounds to further studies on group decision and advices for conservation in group-living animals.