I’ve an unmanaged garden outside my apartment. It used to have several shrubs and pine straw, but the owners ripped them out and now they just trim the weeds every few weeks. They trimmed a week and a half ago, and today I observed these insects swarming on the ground throughout the garden. The largest specimens observed are no more than 3mm long. I am located in south east Georgia, USA, about an hour inland from Savannah.
It appears it may be a Nysius Raphanus (false chinch bug). Quoting from wikipedia:
Nysius Raphanus (false chinch bug) are a small North American insect, no larger than 1/8th inch (3.175mm). They are often grey to brown in color, with largely transparent wings, and can release an offensive odor similar to stinkbugs. They have no larval stage, instead going through several nymph stages with the nymphs resembling adults but having no wings. In high numbers false chinch bugs can cause significant plant damage.