It's an odd grouping in a way. Only two of those fellows are wearing pistol belts. One of those has a whistle chain, so, he's likely the squad/section leader.
The rest are wearing versions of the modified cartridge belts to carry a pistol magazine pocket. Mills wove belts a pockets short; existing belts had a pocket removed, or an extension of regular belt width added on.; or the purpose-made mounted belt.
Given the lack of suspenders, I'm tempted to conclude that this is a cavalry or mounted unit. They could be a mounted MG or field gun unit--the TO&E for those units would include sidearms for all hands.
But, that's just a guess. Most of the "official" issue for US forces in WWI was not actually ready until 1919. The change from mounted infantry to mechanized infantry complicated the neat, orderly, organized QM Tables.