You probably want only C and D in your output since only C and D are on leave, rather than B,C,D? Let me know if that is correct.

Fiddle

SELECT u.userid, u.username, u.useremail, u.userphone, u.specialid
FROM users u
LEFT JOIN absent a
  ON u.userid = a.userid 
  AND CURDATE() BETWEEN a.startdate AND a.enddate  
 WHERE a.userid IS NULL;