Some of my bees got out for cleansing flights a couple days ago and a bunch of them landed on the side of my shed and began scenting, grooming each other, and I saw a little trophallaxis happening too. And it wasn’t a swarm. I’ve only been keeping bees since 2010 (not a long time) with a relatively small number of hives, and I rarely meet up with other beekeepers, so it’s not surprising that I’ve never seen or heard about this before.
My best guess is that the bees have been clustered deep down in their hives all winter, buried under snow for most of it, and they haven’t had a good day for cleansing flights until now. Honey bees communicate and get to know each other by touching (grooming, bumping up to each other) and feeding each other through the exchange of enzymatic fluids in their guts. It’s how the smell of each other and, more importantly, the queen is spread throughout the colony. It’s a big part of how they stay together and work together as a single super-organism. From what I’ve seen, they usually do this kind of getting-to-know-each-other-again-after-a-long-hard-winter socialising inside or near the entrance of the hive. But I guess they were just enjoying the fresh air and sunshine so much, some of them decided to stay outside and others joined in the party.
UPDATE: It’s obvious now what the bees are doing. They’re hanging out in a warm spot of sunshine just like cats do.