Id call them a musical group or 'band'.
An Ensemble This actually depends upon how many people you have, and what kind of instruments they are playing. A large group playing traditional European instruments is called an orchestra. A smaller group playing traditional European instruments is called an ensemble. A group playing more modern music is called a band.
A Duet
Choir, harmony
It is called a choir or a band
A duet
a vocal quartet
The Mormon Tabernacle Choir
A group of five musicians is known as a "quintet."
They can be called an ensemble, a choir, or a group.
Whatever they want it to be they get to choose
Well its not a group if it's only one person. So you would refer to them as a singer.
flautists