the best way, just in case

I don't understand what the risk is. Is there a reason not to allow more latitude for use of judgment when a photo is about a group of specimens and the specimens are clearly labeled as different individuals? When I found aphids and aphid mummies on a single leaf, I posted one photo and asked what was going on there. Flies are bigger than aphids, and all these flies were lined up facing the same way, but if a dorsal-posing fly and a lateral-posing fly had been close enough together, I could have posted one photo that showed both of them. Linking two flies accomplishes the same thing.

I guess if everyone follows the same rules without using their own judgement and latitude then we don't have to read every comment under every image to see if this one isn't really the same specimen.

Ants in a colony are a group, bees in a hive are a group, even aphids are a population group. Moths at a light are not a group (I have seen many species all independantly there for the light), flies on a dead mouse are not a group (I have seen many species on a single mouse all independantly there for the mouse), bees on a flower are not a group (they all independantly came for the flower). I suspect that it is possible for a variety of flies to be on a compost heap. I don't know if that is the case here, it probably isn't, they are probably the same, but see reason one.

I see the distinction. What I found was something like a swarm, and the question was why these identical flies were all suddenly there at once. (I suspect that they hatched together nearby, but there's no way to know for sure.) That is, I believe these two photos show members of a genuine group although the individuals in the group were all perching a few inches from one another by the time I got there. They weren't independently attracted to a compost heap. The compost is in a covered bin. The flies were on various vertical surfaces near it: the front of the bin, the back wall of the garage, the edge of the potting bench, and so forth. I don't usually see craneflies in that area at all.

They were members of a population that behaved and looked alike. I thought saying they were different individuals and emphasizing it typographically would be enough. It's quicker than posting the second photo, getting its number, and returning to the first one to add a thumbnail. Less devouring of bandwidth, too. Other contributors have done similar things with multiple specimens.

I only see 1 anal vein and I don't see a compete V-shaped suture in the thorax, but I don't know what else has the remainder of this fly's wing venation.

too technical for me, but I still wonder why these flies suddenly appeared in such numbers. Did they probably hatch nearby, and they were hanging out because they weren't ready to fly, or because they're diurnal? I didn't think of craneflies because these were so small.

Crane flies don't live long as adults. It is to their advantage to emerge all together, 30 on one day instead of one a day for a month. That increases the chance of finding a mate. I don't know whether that is the explanation for your band of flies. Species of insect vary quite a bit in emergence habits, synchronous or spread out. There are many factors influencing their life cycle.

If they flew away when you approached they were probably ready to fly. I photographed a newly-emerged midge and it was extremely reluctant to fly no matter how close I got.