In a basis of article lays understanding of the nature of a stranger and the group, offered by G. Simmel. Presence of a stranger is one of necessary elements for comprehension and maintenance of unity of group. Main attention of the article is given to a specific character of interactions of group and stranger, to how group supports its own borders and prevents penetration of a stranger into its community. Reasons why group does not accept stranger and why he cannot become a full-fledged member are considered in the article. It is suggested that group has, supports and constructs its own borders, which, on the one hand, limit what is inside the group, its unity and solidarity. On the other hand, borders prevent approaching of stranger with group and its penetration into internal system of interactions. Borders in the article are understood as special spatial limitations or social practices that determine social interactions.