Inte snyggt, men sätter ditt tillstånd direkt i JOIN
borde fungera:
SELECT a.ID, a.join_date, a.country, a.email, b.start_date, b.joined_from
FROM a LEFT JOIN b ON a.country = b.country
AND b.start_date = (
SELECT MAX(start_date) FROM b b2
WHERE b2.country = a.country AND b2.start_date <= a.join_date
);