musical group vs sextet

musical group and sextet both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical group	Yes	No	No	No
sextet	Yes	No	No	No

As nouns, sextet is a hyponym of musical group; that is, sextet is a word with a more specific, narrower meaning than musical group:

musical group (noun)	sextet (noun)
an organization of musicians who perform together	a musical composition written for six performers
	six people considered as a unit
	a set of six similar things considered as a unit
	six performers or singers who perform together
	the cardinal number that is the sum of five and one