The choir.
A singing group of eight is called an octet.
Choir
A 'Choir' are a group of singers singing songs. Therefore a church choir are those members of the congregation who come together to practice singing the church music so that they can lead the congregation in singing praises to god.
Quartet.
A acapella 5 man singing group would be called a Barber shop choir.
Carolers.
Trio
A singing group of three is called a "trio."
The group of people in the church are called a congregation.
It's called a quintet
A quartet is a singing group of four.
It's called a Quartet. :D