Who is the Narrator at the very beginning of the Prologue? Who joins him, and for what purpose?

"When April comes with his sweet, fragrant showers, which pierce the dry ground of March, and bathe every root of every plant in sweet liquid, then people desire to go on pilgrimages." Thus begins the famous opening to The Canterbury Tales. The narrator (a constructed version of Chaucer himself) is first discovered staying at the Tabard Inn in Southwark (in London), when a company of twenty-nine people descend on the inn, preparing to go on a pilgrimage to Canterbury. After talking to them, he agrees to join them on their pilgrimage. From here on, we also hear the voices of those on the journey, the pilgrims.