Group behavior involves two or more people interacting as members of a group to achieve shared goals. There are two main types of groups: formal groups, which have defined rules and goals; and informal groups, which are based on friendship and personal relationships. People join groups for security, affiliation, power, goal achievement, self-esteem, and status. As groups develop over time, they establish norms, roles, status hierarchies, and levels of cohesion. Group decision-making processes aim to leverage the knowledge and talents of members through techniques like brainstorming and nominal group processes.