Formal:
* Ensemble - This is a general term for a group of musicians performing together, especially if they are playing instrumental music.
* Orchestra - This is a large group of instrumentalists, typically playing classical music.
* Band - This is a term for a group of musicians, often playing popular music, but can also be used for groups playing classical music or other genres.
Informal:
* Group
* Team
* Section (for a specific group of instruments within a larger ensemble)
* Players
Ultimately, the best collective noun to use depends on the specific context and the type of music being played.