Informal organizations are groups of people formed on the basis of feelings, personalities, and similar interests in joint work or mutual contact. These groups are not restricted by the administrative department and management level of formal organizations, and there are no formal formal regulations. Structure, but some specific relationship structures are formed within it, and their own "leaders" naturally emerge, forming some unwritten codes of conduct and norms. According to the above definitions, Which of the following groups of people are informal organizations:. ["Pharaoh's birthday, invited more than ten colleagues to party in his home.", 'Xiao Liu plays football with several young people in his unit after work.', 'Xiao Zhang is the head of the arts department of the student union of the school. After his introduction, several students participated in the performances.', 'Several retired employees spontaneously set up a folk art club for the elderly to perform in the community on weekends.']