founder member
noun [ C ] UK uk/ˌfaʊn.də ˈmem.bər/ us/ˌfaʊn.dɚ ˈmem.bɚ/ (US founding member)
（机构或摇滚乐队的）创始成员之一，发起人之一
one of the original members of an organization or rock group
France is a founder member of the European Union. 法国是欧盟的创建国之一。