You can get more data from that query by selecting for more fields from the topic_users table. To find whether the user liked a post, or posted in the topic, try this:
-- [params]
-- null string_list :topic_id
WITH user_data AS (
SELECT u.username, ue.email, tu.topic_id, tu.posted, tu.liked FROM users u
LEFT JOIN topic_users tu ON u.id = tu.user_id AND tu.posted = true
LEFT JOIN user_emails ue on u.id = ue.user_id
)
SELECT * FROM user_data WHERE topic_id = :topic_id