Quartet.
A group of four people singing is called a quartet.
Unified timbre is another meaning of the word harmony. When all persons singing, sing in harmony, the song is by far more pleasant to the listener.
Singing
This is rpatel and D7lan... Another word for singing is t-paining :P
A chorus is a singing group. Other singing groups include a trio.
Another word for a singing group is "vocal ensemble." This term encompasses various types of groups that perform together, often highlighting harmonies and vocal arrangements. Other alternatives include "chorus" and "a cappella group."
persons abilities
Another word for a singing group or choir is "vocal ensemble." This term encompasses various arrangements of singers who perform together, often specializing in harmonies and choral works. Other synonyms could include "chorus" or "glee club," depending on the context and style of the group.
it is called singing class
chorus
Another word for sitting together could be lounge or gather.
Another word for grin teeth together would be grit.