Building a sizable software project never a simple or straightforward task and often requires more than one person to complete the mission. As a project grows, domain, product, and technical knowledge are spread out across many members of a team. "XP acknowledges this reality by creating cross-functional teams composed of diverse people who can fulfill all the team's roles".
Other than scheduled activities, everyone on the team plans his own work. Self organization is the hallmark of agile teams.