Formal:
* Chorus: This is the most common and widely understood term, especially for a group of singers performing together.
* Ensemble: A more formal term, suggesting a group with a high degree of coordination and skill.
Informal:
* Choir: This is a specific type of singing group, often associated with religious music.
* Glee club: This is a term for a school or college singing group, often known for their lighthearted and energetic performances.
* Group: This is a very general term, suitable for any group of people singing together.
* Crowd: This is a more casual term, often used for a large group of people singing together spontaneously.
You could also use a descriptive phrase like "a group of singers" or "people singing in harmony". Ultimately, the best term will depend on the specific situation and the desired level of formality.