All of them are wearing breeches with leggings, which look like what was issued in 1917 after the M1910 Leggings were replaced. They may be a bit shorter than the 1898 leggings. M1908 & M1910 Breeches have regular belt loops - we have post cards in our collections from the 1912 - 1916 period showing RA troops wearing personally owned belts on their breeches. The hat style is either Span-Am period or M1911 Peaked hat changed to look like a hat with just the center crease, hard to tell from the picture, but I lean towards the former. The cook is probably wearing denim work clothing. In the front row, there are two officers, both with the leather officer's leggings of the WW I style and wearing ties. To the left of the bugler, you can see the man in the front row is wearing a marksmanship badge. Without being able to see the detail on the shirt, it is not possible to tell if they are wearing S-A wool shirts with full button fronts or the later 3 button M1910 wool shirt. My instinct goes with the blue wool shirt. Probably Post S-A National Guard prior to the July 1, 1916 reorganization of the Army at all levels.
One other thought - There were 49 Battalions of United States Guards raised between January 1918 until their demobilization in February of 1919. Generally used for guarding government installations. Made up of men unable to serve overseas or those who were possibly exempt from the draft.* * Order of Battle of the US Land Forces in the World War (1917-19) Zone of the Interior Directory of Troops in Alphabetical Order. GPO Washington, D.C. 1949 Volume 3 Part 2 PP 1566-1570.