As a woman, my experience says this is largely dependent on the culture. Go to Latin America and men cat call women even when they're alone. Day, night, at work- doesn't matter. If anything, I've never been catcalled in enclosed spaces such as public transport or elevators, but I've occasionally had a hand on my ass if it's super crowded. If you ask me this totally puts them in the bully category, and bullies can also work in groups.

In Australia I've only seen men catcalling women while they are drinking, and yeah they were also not drinking alone. The catcalling also feels sillier or less serious somehow, perhaps because it's obvious they're being drunk and stupid.

YMMV.

Edit: also let me add, in the first example men catcall any woman. I think any woman between the ages of 12 and 60 is a good target for catcalling, as long as they're not morbidly obese.

In the second example, only conventionally attractive women, often dressed somewhat suggestively, get catcalled.