The Gulf Cooperation Council has had the same six members since 1981
The Gulf Cooperation Council, formally the Cooperation Council for the Arab States of the Gulf, is a union of six Arab monarchies on and around the Arabian Peninsula. They signed its charter on 25 May 1981, and the membership has not changed since. The council launched a customs union in 2003 and a common market in 2008. All six members produce oil, but they stand in different places with OPEC: some are members, some have left, and some have never joined. Neighbours that have been offered membership, or have taken part in some of the council's work, are not members.
Name them all first. Then try the harder part: sort them by their link with OPEC.