Here are some options you could use, depending on the context:
General options:
* A crowd of onlookers
* A gathering of onlookers
* A throng of onlookers
* A group of onlookers
* A bevy of onlookers (implies a large, diverse group)
More specific options:
* A gallery of spectators (if they are watching a performance)
* An audience of viewers (if they are watching something on a screen)
Ultimately, the best collective noun to use will depend on the specific situation and the desired tone.