SELECT first_name, last_name, middle_name, email, phone FROM userman
WHERE MONTH(datereg) = N
/* вместо N поставить нужный месяц
либо для текущего месяца MONTH(NOW()) или MONTH(CURRENT_DATE())
/*

/*ПН 00-00 часов*/
ADDTIME(DATE_SUB(DATE(NOW()), INTERVAL WEEKDAY(NOW()) DAY),)
/* ПТ 16-00*/
ADDTIME(DATE_SUB(DATE(NOW()),INTERVAL 4-WEEKDAY(NOW()) DAY),'16:00:00')
/* получаем */
SELECT first_name, last_name, middle_name, email, phone FROM userman
WHERE datereg  BETWEEN
  ADDTIME(DATE_SUB(DATE(NOW()), INTERVAL WEEKDAY(NOW()) DAY),) AND
  ADDTIME(DATE_SUB(DATE(NOW()),INTERVAL 4-WEEKDAY(NOW()) DAY),'16:00:00')