The problem is you're subtracting the joined date from the current date and seeing if it is EQUAL to 9. That will only ever match a single date. In your example, that would be everyone who joined only on 6/21, or (6/30 - 6/21 = 9).
If you're looking for everyone who joined in the last 9 days, you have to use a different comparison operator, less than or equal to, or <=.
SELECT * FROM users where (DATEDIFF(CURDATE(),joineddate) <= 9)