You're there to record video, you point your camera at some dancing guests, and they notice you -- so they stop and pose.
If this were a video game, maybe the action would pause and you'd get a choice of what to do.
Option 1: Yell out, "I'm doing video, not photo" over the dance music.
Result: this makes things awkward. For instance, there's a strong possibility that they don't hear you. So, they just keep on posing, waiting for the flash.
Option 2: Walk in close, put your mouth to their ear, and explain.
Result: even more awkward. Takes up more time. They need to put their mouths to their friends' ears to explain; and then, as blogger Allie Brosh would put it, "We just stood around feeling weird about ourselves and each other."
Option 3: Pretend you're taking a picture, then give a thumbs up.
Result: they look confused as to why no flash went off, but usually they just smile and carry on.
You probably can't use the shot in the edit (but sometimes you can -- it just depends).
As most videographers have learned, this is the easiest way out.
And if you're still standing there recording them afterwards, they'll feel weird, but they won't ask for a second photo.
Option 4: Just take a photo anyway (if your camera can do it).
Result: you get a pointless photo.
Option 5: Open your shirt to expose your T-shirt. I've actually printed out a T-shirt that says, "Videographer".
Result: sometimes they laugh and go back to dancing, and sometimes the reaction can be, "Video? Yuck!", and then they turn their backs on you. Many people don't want their dance moves preserved for posterity.
