What is the collective nouns of onlookers?

There isn't a single, universally accepted collective noun for onlookers. However, here are some options that work well depending on the context:

Formal:

* Assembly

* Audience

* Spectators

Informal:

* Crowd

* Gathering

* Group

Descriptive:

* A throng of onlookers

* A sea of faces

* A curious bunch

Ultimately, the best collective noun will depend on the specific situation and the desired tone.