orchestra vs musical group

orchestra and musical group both are nouns.

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

As nouns, musical group is a hypernym of orchestra; that is, musical group is a word with a broader meaning than orchestra:

orchestra: a musical organization consisting of a group of instrumentalists including string players
musical group: an organization of musicians who perform together

Other hypernyms of orchestra include musical organisation, musical organization.

orchestra (noun)	musical group (noun)
seating on the main floor in a theater	an organization of musicians who perform together
a musical organization consisting of a group of instrumentalists including string players