More than one person in a group may show leadership but each instance of leadership has to be to take the group in a slightly different direction or to make a difference in how the group is thinking. If one person simply echoes the original leader then that is followership. So, leadership may shift from one person to another in short order within a group, but each instance of leadership is unidirectional not participative. You have to occupy one position or the other at any one moment in time. Leadership is about advocating a change in direction, management is about executing that direction as efficiently as possible.
Some leaders influence by being charismatic.
Others clearly express a vision. This sounds like I am saying that the group never influences the leader. No, I am not saying that. I am saying that each instance of influence in a group is unidirectional. The ultimate direction taken may be a joint decision but it will be a result of a number of people exerting their leadership individually in the discussion.