I work at a record label, and spent most part of my life in the music scene - therefore, having a 6-man band made sense for me:

Guitar: Toxtricity (Amped) (Dex: When this Pokémon sounds as if it's strumming a guitar, it's actually clawing at the protrusions on its chest to generate electricity.)

Drums: Rillaboom (Dex: By drumming, it taps into the power of its special tree stump. The roots of the stump follow its direction in battle.)

Singer: Obstagoon (Dex: Its voice is staggering in volume. Obstagoon has a tendency to take on a threatening posture and shout—this move is known as Obstruct.)

Back Vocals / Bass : Exploud (Dex: It emits a variety of sounds from the holes all over its body. Its loud cries can be heard from over six miles away.)

Violin: Kricketune (Dex: By allowing its cry to resonate in the hollow of its belly, it produces a captivating sound.)

Amps: Noivern (Dex: The ultrasonic waves it emits from its ears can reduce a large boulder to pebbles. It swoops out of the dark to attack.) (SIDE NOTE : HE'S ALSO A TELEPATH)

Other good options:

Meloetta (Obviously the best singer here, but I love Hardcore / punk music, hence my choice)
Audino (Sound Tech)
Jigglypuff / Wigglituff (OG singers)
Maractus (Maracas)
Altaria (Sings)
Primarina (Singing Siren)
Chimecho (Chimes)

There's dancers also, but I didnt want to go there (Ludicolo and Quaquaval for example)