The passages that you are looking for are right in the first page or two of chapter 30. Boo Radley, who has spent 30 years of his life living in his house where no one but his family has seen him, has been launched into public view by being in Scout's house. He is out of his comfort zone, and in the living room where he is at the beginning of chapter thirty, everyone can see him clearly, and scrutinize him. Imagine if you had been holed up in your house for 30 years, never seeing the light of day or any other place, finding yourself all of a sudden, and without preparation or even willingly choosing to, being launched into a strange house, a strange room, with all the lights on. You would feel nervous, conspicuous, and self-conscious.
Atticus, being sensitive to Boo's potential and understandable social anxiety, suggests they all go out on the porch; Scout understands it is because "the livingroom lights were awfully strong" and Boo could sit there relatively in the shade, unobserved, which would much more comfortable for him. So, they file out the door, and soon it is only Boo and Scout left. Boo is very nervous; he isn't sure what to do, and is too scared to move. He hasn't been in a situation like this for most of his life, and to boot, he is in a house he has never been in, so he doesn't know his way around. So, Scout jumps in to help guide him:
"Come along, Mr. Arthur,...you don't know the house real well. I'll just take you to the porch, sir."
So, Atticus suggests they move to the porch to be sensitive to Boo's situation, and to make him feel more comfortable, and Scout leads him out there because he is nervous and unsure of himself. Plus, she has been trained to be polite, and it was the polite thing to do. I hope that helps!