noun
member nation; member state; signatory
Refers to a country that has formally joined an international organization, treaty, or alliance.
日本は国連の加盟国です。
Japan is a member state of the United Nations.
新たな加盟国が条約に署名した。
A new signatory nation signed the treaty.
参加国 means a participating country, often in an event or project, whereas 加盟国 implies formal membership in an organization.
Compound of 加盟 (かめい, 'joining, affiliation') and 国 (こく, 'country').