UN Charter, Chapter II, Articles 3-6 concern Membership in the UN. Only states can be members of the United Nations.
Membership in different organs of the UN varies.
Membership of the principal organs is determined by the Charter of the United Nations and the Statute of the International Court of Justice:
In accordance with the Charter, the General Assembly (Article 22), Security Council (Article 29) and the Economic and Social Council (Article 68) may establish subsidiary organs.The decision to establish a subsidiary organ (also called the mandate) usually includes:
Websites, research guides, and FAQs with more information.
The Charter of the United Nations / 
Oppenheim's International Law /
The United Nations, whose specialized agencies were the subject of an Appendix to the 1958 edition of Oppenheim's International Law: Peace, has expanded beyond all recognition since its founding in 1945.This volume represents a study that is entirely new, but prepared in the way that has become so familiar over succeeding editions of Oppenheim. An authoritative and comprehensive study of the United Nations' legal practice, this volume covers the formal structures of the UN as it has expanded over the years, and all that this complex organization does. All substantive issues are addressed in separate sections, including among others, the responsibilities of the UN, financing, immunities, human rights, preventing armed conflicts and peacekeeping, and judicial matters. In examining the evolving structures and ever expanding work of the United Nations, this volume follows the long-held tradition of Oppenheim by presenting facts uncoloured by personal opinion, in a succinct text that also offers in the footnotes a wealth of information and ideas to be explored. It is book that, while making all necessary reference to the Charter, the Statute of the International Court of Justice, and other legal instruments, tells of the realities of the legal issues as they arise in the day to day practice of the United Nations. Missions to the UN, Ministries of Foreign Affairs, practitioners of international law, academics, and students will all find this book to be vital in their understanding of the workings of the legal practice of the UN. Research for this publication was made possible by The Balzan Prize, which was awarded to Rosalyn Higgins in 2007 by the International Balzan Foundation.
The Procedure of the UN Security Council / 
United Nations Handbook / Chinese: 联合国会员国
English: UN Membership
French: Les membres de l'Organisation des Nations Unies
Russian: Членство в Организации Объединенных Наций
Spanish: Miembros de la ONU
In the context of the Charter, membership and participation in the UN are different.
UN Charter, Chapter II, Articles 3-6 concern Membership in the UN. Only states can be members of the United Nations.
Membership in different organs of the UN varies.
Articles 31-32 of the Charter concern participation in the Security Council by states.
Articles 69-71 of the Charter concern participation in the Economic and Social Council by states, specialized agencies and non-governmental organizations.
In addition, the General Assembly has an established practice of granting observer status to non-member states and intergovernmental organizations, see decision 49/426 in A/49/49 (Vol. I), page 341. Entities with observer status receive "a standing invitation to participate as observers in the sessions and the work of the General Assembly" (A/INF/73/5).
To research these matters further, consult secondary sources about the organ or the article of the Charter. For example, the Repertoire of the Practice of the Security Council has a section on the Council's role in membership as well as section on participation in the Council.
Regional groups in the UN are groups of states that, among other discussions, coordinate nomination of candidates for election to various bodies.The major regional groups are the Group of African States, the Group of Asian States, the Group of Eastern European States, the Group of Latin American and Caribbean States, and the Group of Western European and Other States.
The Journal of the United Nations announces the monthly chairs of the regional groups, as well as a wealth of other information.
