According to shared leadership approaches, leadership is a dynamic, interactive influence process among individuals in groups for which the objective is to lead one another to the achievement of group or organizational goals, or both.27 This influence process occurs both laterally—among team members—and vertically, with the team leader. Vertical leadership is formal leadership; shared leadership is distributed leadership that emerges from within team dynamics. The main objective of shared leadership approaches is to understand and find alternate sources of leadership that will impact positively on organizational performance.

Shared leadership is a dynamic, interactive influence process among team members working to achieve goals.

In shared leadership, leadership can come from outside or inside the team. Within a team, leadership can be assigned to one person, rotate across team members, or be shared simultaneously as different needs arise across time. Outside the team, leaders can be formally designated. Often these nontraditional leaders are called coordinators or facilitators. A key part of their job is to provide resources to their unit and serve as a liaison with other units. 
According to the theory, the key to successful shared leadership and team performance is to create and maintain conditions for that performance. This occurs when vertical and shared leadership efforts are complementary. Although a wide variety of characteristics may be important for the success of a specific effort, five important characteristics have been identified across projects: (1) efficient, goal-directed effort; (2) adequate resources; (3) competent, motivated performance; (4) a productive, supportive climate; and (5) a commitment to continuous improvement.28 The distinctive contribution of shared leadership approaches is in widening the notion of leadership to consider participation of all team members while maintaining focus on conditions for team effectiveness. 

In-text citation:  (Uhl-Bien, Osborn, & Schermherhorn, 2014, p. 296). 

What is followership?

Followership represents a process through which individuals choose how they will engage with leaders to co-produce leadership and its outcomes.
Romance of leadership is the tendency to attribute organizational outcomes (both good and bad) to the acts and doings of leaders; its corollary is the “subordination of followership.”
The social construction of followership shows that followers hold beliefs about how they should act in relation to leaders, but whether they can act on these beliefs depends on context.
Those with power distance orientation accept that power in institutions and organizations is distributed unequally, whereas those with proactive follower orientations believe followers should act in ways that are helpful and productive to leadership outcomes.
Implicit followership theories show managers’ views of characteristics associated with effective and ineffective followership.

What do we know about leader–follower relationships?

Leader–member exchange theory shows that managers have differentiated relationships with subordinates depending on the amount of trust, respect, and loyalty in the relationship.
These relationships are important because they are differentially related to leadership and work outcomes. When relationship quality is high, performance is better, subordinates are more satisfied and supported, commitment and citizenship are higher, and turnover is reduced.
Relationships develop through processes of social exchange based on the norm of reciprocity (i.e., when one party does something for another, an obligation is generated until it is repaid).
Reciprocity is determined based on three components: equivalence (whether the amount given back is same as what was received), immediacy (how quickly the repayment is made), and interest (the motive behind the exchange).
Idiosyncrasy credits mean that when we have enough credits built up in relationships with others, we can get away with idiosyncrasies (i.e., deviations from expected norms) as long as the violation does not exceed the amount of credits.

What do we mean by leadership as a collective process?

Collective leadership advocates a shift in focus from traits and characteristics of leaders to a focus on the shared activities and interactive processes of leadership.
Distributed leadership sees leadership as drawing from the variety of expertise across the many, rather than relying on the limited expertise of one or a few leaders.
Co-leadership is when top leadership roles are structured in ways that no single individual is vested with the power to unilaterally lead.
Shared leadership defines leadership as a dynamic, interactive influence process among individuals in groups for which the objective is to lead one another to the achievement of group or organizational goals, or both.
Shared leadership occurs both laterally, among team members, and vertically, with the team leader. The main objective is to understand and find alternate sources of leadership that will impact positively on organizational performance.