The narrator joins the group of pilgrims as a result of
chance or pure accident
the intervention of the innkeeper
arrangements made through the church
pre-existing friendships with the other pilgrims

In the Canterbury tales, the narrator joined the group of pilgrims as a result of chance. He met the company of pilgrims when staying at a tavern at Southwark on his way to Canterbury, the company agreed to let him join them since they are going to the same place.