This tiny fly species was abundant at a Peninsula College site where I was studying digging bees.
I shot a short video of the site to show the amount of activity. On viewing it at home, I saw two specks enter the frame from the left, with one closing on the other. I made a 1 frame/second movie from the original, with the flies denoted by arrows (6).
Both flies fell to the ground, rolling around for several seconds. (You can see a much larger hymenopteran fly to the right at the end of the clip.) I switched to photo mode and got some stills of the pair, then caught one of the pair and photographed it at home with a dissecting microscope (5).
This is a Conopid fly, possibly in the genus Thecophora. (See Thecophora on bugguide.net.) Conopid flies lay eggs on hymenopterans, so this species is probably congregating on the site to gain access to the various hymenopterans there. Conopid flies waylay their hosts in flight, laying an egg that matures into an internal parasitoid that eventually kills the host.
This pair may have been mating. I observed similar behavior on subsequent visits to the site.