As a founding member of the club, he has witnessed its entire development.

founding 是动词 found（建立、创立）的现在分词形式，这里用作形容词，表示"创始的"。 member 的意思是"成员"，指某个组织或团体中的一员。 founding member 指的是"创始成员"，即在某个组织或团体最初成立时就加入的成员。