Student or Learner
"The National Guard of the United States is a joint activity of the United States Department of Defense composed of reserve components of the United States Army and the United States Air Force."
The National Guard is a group of people, not an activity. Could this be an error?
Not an error, no. It might not be the most obvious or clear choice of words, but look at it like this "Reserves from The United States Army and The United States Air Force act together to form The National Guard". They act together so forming the National Guard is a joint activity.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.