Thanks Ray, with a slight rewrite I got it to work as follows:
SELECT *, (SELECT SUM(Minutes) FROM CPD AS u2 WHERE u2.User_ID = users.ID AND YEAR(DATE) = 2019) AS MinuteCount FROM users LEFT JOIN CPD on users.id = CPD.User_ID GROUP BY users.display_name ORDER BY users.display_name ASC
and then I displayed it on the page in hours as follows:
<?php echo round($rsUsers->getColumnVal("MinuteCount")/60,2); ?>