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.
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.
According to the UN Charter, Article 23, "The Security Council shall consist of fifteen Members of the United Nations"
The Security Council has 15 members:
In 1963, the General Assembly recommended an amendment to the Charter to increase the membership of the Security Council
According to UN Charter, Article 61, "The Economic and Social Council shall consist of fifty-four Members of the United Nations elected by the General Assembly".
The Economic and Social Council has 54 members:
The Charter has been amended twice to increase the membership of the Economic and Social Council
In the context of the Charter, membership and participation in the UN are different. Articles 31-32 concern participation in the Security Council by states. Articles 69-71 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 states and intergovernmental organizations, see decision 49/426 in A/49/49 (Vol. I), page 341.
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.