grass roots vs common people

grass roots and common people both are nouns.

Word	Noun	Adjective	Verb	Adverb
grass roots	Yes	No	No	No
common people	Yes	No	No	No

As nouns, common people is a hypernym of grass roots; that is, common people is a word with a broader meaning than grass roots:

grass roots: the common people at a local level (as distinguished from the centers of political activity)
common people: people in general (often used in the plural)

Other hypernyms of grass roots include folk, folks.

grass roots (noun)	common people (noun)
the common people at a local level (as distinguished from the centers of political activity)	people in general (often used in the plural)
the essential foundation or source