Why do you think boo"joined" the community that night? What was his purpose and what does it say about his character? How might our views of his identity have changed?

I'm thinking that you mean the night of the fire. Boo really didn't join the community. He kind of just lurks around in the darkness. He sees Scout who he has taken a sort of watchful liking to. He places a blanket over her shoulders and disappears. Certainly we begin to see Boo as a "malevolent phantom" rather than a scary monster.