Choir, harmony
Id call them a musical group or 'band'.
It is called a choir or a band
A duet
a vocal quartet
A group of singers is known as a choir.
The Mormon Tabernacle Choir
The singers in a vocal group are divided according to their height.
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.
A group of 5 singers is typically called a quintet, a group of 6 singers is called a sextet, a group of 7 singers is called a septet, a group of 8 singers is called an octet, and a group of 9 singers is called a nonet. These terms are commonly used in music to describe vocal ensembles of varying sizes.
A group of four singers is also known as a quartet.