== Trio== ] ==
Three people singing together is called a trio. It is also called like that for instrumentalists!
A sextet.
A duet.
Choir
Quartet.
Three people singing together is called a trio. It is also called like that for instrumentalists!
A duet.
Trio
A sextet.
Choir
Quartet.
two people singing together is a duet
The general term would be a quintet.
The word you might be looking for is Kabuki.
Counterpoint
Audio engineer or sound engineer
While there are several different types of singing without words, a common one is called scat singing. This type started in jazz and has transcended to many different genres.