Basically, for a band on a typical song, you can have three to five parts, typically: percussion, bass guitar, guitar, second guitar, and keyboard.
So add in the singer, and you get potentially six parts, if each is devoted. (Could also be more if backup singers are featured, or other instruments like flutes, violins, cellos, saxophones, trombones, or trumpets. Some of which are becoming increasingly popular as well.)
So the lineup as it currently stands is that Ruby is handling percussion. (She can handle any instrument, but percussion is her preferred instrument.)
D.D. is handling bass guitar.
Gary is both the singer and lead guitarist.
Whitney has a similar talent to Gary and is his backup singer as a result, while also handling whatever instrument needs to be handled. (She can play any instrument, similar to Ruby.)
And I think that Vili handles the second guitar, with Amy maybe handling the keyboard.
Which leaves Sally and Hannah as managers/producers for the band. (Dion funds them.) Dale along with Herald act as security. Meaning the whole Ruby Gang is involved, even if the band can potentially get by with as little as four members. (Ruby, Gary, and Whitney can handle anything even if they have their preferences, leaving them almost covered for most songs.)
I think it's a fairly nice idea, even if it's not exactly much of a plot point.