SELECT user_id
FROM user_movies
GROUP BY user_id
ORDER BY COUNT(
    movie_id IN (SELECT movie_id FROM user_movies WHERE user_id = 305344)
    ) DESC
LIMIT 50

SELECT inst1.user_id
FROM user_movies inst1, user_movies inst2
WHERE inst1.user_id != inst2.user_id
  AND inst1.movie_id = inst2.movie_id