Who is the narrator of the ballad? To whom is he telling his story?

In the poem's first line, we meet the protagonist and narrator, "an ancient Mariner." He stops one of three people on their way to a wedding celebration. The leader of the group, the Wedding Guest, tries to resist being stopped by the strange old man with the "long grey beard and glittering eye." He explains that he is on his way to enjoy the wedding merriment; he is the closest living relative to the groom, and the festivities have already begun. Still, the Ancient Mariner takes his hand and begins his story. The Wedding Guest has no choice but to sit down on a rock to listen.