Register User Taxonomy

add_action( 'init', 'my_register_department_taxonomy' );
/**
 * Registers the 'department' taxonomy for users.  This is a taxonomy for the 'user' object type rather than a
 * post being the object type.
 */
function my_register_department_taxonomy() {

	 register_taxonomy(
		'department',
		'user',
		array(
			'public' => true,
			'labels' => array(
				'name' => __( 'Departments' ),
				'singular_name' => __( 'Department' ),
				'menu_name' => __( 'Departments' ),
				'search_items' => __( 'Search Departments' ),
				'popular_items' => __( 'Popular Departments' ),
				'all_items' => __( 'All Departments' ),
				'edit_item' => __( 'Edit Department' ),
				'update_item' => __( 'Update Department' ),
				'add_new_item' => __( 'Add New Department' ),
				'new_item_name' => __( 'New Department Name' ),
				'separate_items_with_commas' => __( 'Separate departments with commas' ),
				'add_or_remove_items' => __( 'Add or remove departments' ),
				'choose_from_most_used' => __( 'Choose from the most popular departments' ),
			),
			'rewrite' => array(
				'with_front' => true,
				'slug' => 'author/department' // Use 'author' (default WP user slug).
			),
			'capabilities' => array(
				'manage_terms' => 'edit_users', // Using 'edit_users' cap to keep this simple.
				'edit_terms'   => 'edit_users',
				'delete_terms' => 'edit_users',
				'assign_terms' => 'read',
			),
			//'update_count_callback' => 'my_update_user_term_count' // Use a custom function to update the count.
		)
	);

}