My initial reaction is: are they insane? Don't they know there is a pandemic going on??????
But maybe they're not insane; maybe the exercise and mental health benefits outweigh the risk. A few days ago I stumbled across some tools that let me do a rough back-of-the-envelope on the size of the risks, and I think we should encourage a lot more physically-distanced outdoor dancing (and singing and yoga and drumming and whatever else makes people happy).
Here's how I figure it:
If there are twenty people getting together to dance, there is about a one percent chance one or more of them have COVID-19 and are infectious but don't have symptoms. That's based the current infection rate of Hampshire County, Massachusetts (where I live) and calculated by this Coronavirus Risk tool.
If somebody is infected, what are the chances they'll spread it to somebody else in the group?
Assuming they all stay at least six feet apart so they're not breathing directly on each other (no large-droplet transmission of the virus), I can use a handy spreadsheet created by a chemistry professor who is an expert on air pollution to get an order-of-magnitude estimate for that risk. That is another one percent chance.
So the chances that somebody in the group catches COVID is the one percent chance somebody is infected, multiplied by the one-percent chance the infection spreads. Or a one in ten-thousand chance for somebody in the group to catch it, or one-in-190,000 individual chance.
Those are very small risks. To put those numbers in perspective, the average 75-year-old male in the US has about a one in ten-thousand chance of dying on any given day.
If you live in a county with a high infection rate... the risk will be much higher (e.g. in George County Mississippi right now the risk would be 1-in-300 somebody in the group would get infected). If you live in Mississippi, you should stay home as much as possible until infection rates fall.
If it is a larger group getting together... the risk would be much higher. Smaller group, much smaller risk.
Dance inside... higher risk, depending on the size of the space and how much fresh or sanitized air flows through it.
So, unless you live in a place where the virus is raging: go outside. Do something with a few other people; keep your distance, and be happy.