Are they dangerous? Why would they be out at night?

If you take a look at the guide page John linked to in his response, you'll see it says that they "come to lights." Lots of insects are attracted to lights at night. (Think of the old expression "like moths to a flame"). So if you don't mind turning off the porch light and pulling down your shades, you can sit outside at night and it will be a lot less "buggy." Can these sting? Yes. Are they likely to sting you? No. They are only there because they are being drawn in, helplessly, by your lights. They are sort of hypnotized and are unlikely to particularly even notice you are sitting there. If you try to swat or spray them, they will be (understandably) stressed, and you're more likely to accidentally get stung. But if you just leave them alone they won't bother you, and if you turn out the lights they'll go away and you'll be able to see a few more stars. (:

Thank you!