What is this flying bug in Denver, CO?
These are Scuttle Flies (Order Diptera, Family Phoridae) (the wing venation, best seen on the lower left individual of the top photograph, is characteristic); the group has several hundred species in America north of Mexico.  So far as I am aware, the generic key in Manual of Nearctic Diptera, Volume 2 (available online here, pages 25 to 48) will still yield useful results, but cannot be used on these photographs.  The 30 pages in bugguide.net's gallery (starting here) eliminate a few of these, but not nearly enough to go further on the evidence we have.