What are the advantages/disadvantages of staying within a small circle of people and never going outside it for any reason?

there are no advantages to that

The advantages, you feel safe. You know everyone, and nothing is unknown (things outside your town or group don't count, as they don't affect you directly).  It may, as you suggest, stunt your growth intellectually, . I cannot say about emotionally. I don't know why you included physically, unless you were thinking of a group that did nothing by sit around the table/computer and talk.
 The Disadvantages, you never see what others are doing. You never experience strange new things. You never see other cultures, and cannot understand the cultures of others if they are spoken about. I have been to 4 continents, (and wish I could travel in the other 3)  Been in numerous countries and while there, toured and interacted with the peoples of those countries. The experiences cannot be fully described.
   But it can be scary, and that is what some people are afraid of. They do not want to leave their Safety Zone. Fear of the unknown.