Adding users to subaccounts is a slightly convoluted process. There's not a place to check a box or add a user. Instead, Canvas adds a user to a subaccount when they are the teacher of a class inside that subaccount. This works out well, because most folks will fall into this category by default. If not, the easiest solution is to add them as a teacher to a course in your subaccount. If necessary, you can manually create a course, make them the teacher, and then manually add the course to your subaccount.