What's really happening when Atticus moves back toward the porch & the crowd draws in? What do they mean to do in To Kill a Mockingbird?
There were lots of 'crowds' or mobs in that area of the book. There were Atticus' friends that came to the house, then they circled up with him again at the church, and a different mob surrounded him at the jail.
The way you have asked this question and used the word porch leads me to believe you are talking about the first group of friends that came to the house. These included the town doc, the sherriff and others who cared about Atticus. Their intention was to tell him, as friends, it might be a good idea for him not to necessarily work hard to defend Tom as it might reflect poorly on Atticus.
Jem and Scout were looking through the windows and screen of the porch so it could be that the mob got closer knowing the kids could hear and the language they used was likely not fit for children in terms of content. When people have something important to share, they often use their gestures to highlight that importance. I would note those features out of the crowd moving closer if this indeed was the circumstance to which you referred.
In this scene (I am assuming you are talking about the scene in Chapter 15) the crowd wants to lynch Tom Robinson. They are so angry about what they believe he has done that they do not want to wait for a trial. This was something that happened relatively often to black men accused of sexually molesting white women in the South during the early 20th century.
If Atticus had not stopped them (with help from Scout) the crowd would presumably have killed Robinson. However, Scout sort of wakes them up to their morals by talking to Mr. Cunningham. By doing so, she reminds him, and them, that they are individuals and not just a mob.