Sentence:  Nor have we remembered that in the melting pot of America the hundreds of isolated and semi isolated ethnic , regional and occupational <WSD>groups</WSD> did not fuse into a homogeneous national unit until long after education and industrialization had caused them to cast oral tradition aside as a means of carrying culturally significant material . 
Meaning of the word marked with <WSD> tags:  any number of entities (members) considered as a unit