LEFT JOIN not joining the second table
After clarification in the fiddle you provided, the only other way I can interpret your issue is you're saying you're not seeing the columns for the users table even though you joined to it. That's because you didn't list any columns from the users table in the SELECT list.
You can add any or all columns from your users subquery you want to the SELECT list like so:
SELECT reservations.*, IF(DATE_FORMAT(reservations.time_from, '%Y-%m-%d') = CURDATE(), IF(reservations.user_id IS NULL, TRUE, FALSE), TRUE) AS removable,
    users.*
FROM reservations
LEFT JOIN (
    SELECT users.id AS userID, users.name AS name, users.surname AS surname FROM users
) AS users ON reservations.user_id = users.userID
WHERE reservations.time_from &gt;= CURDATE()
ORDER BY reservations.time_from; 

Note you shouldn't use * in your SELECT list as it's bad practice and can lead to dependency issues later on, and rather you should explicitly list the column names you want.
Furthermore it seems like you might've gotten a little confused because you used a subquery to join in the users table but that's not necessary, rather you can re-write your query simpler like this:
SELECT reservations.*, IF(DATE_FORMAT(reservations.time_from, '%Y-%m-%d') = CURDATE(), IF(reservations.user_id IS NULL, TRUE, FALSE), TRUE) AS removable,
    users.id AS userID, users.name AS name, users.surname AS surname
FROM reservations
LEFT JOIN users
    ON reservations.user_id = users.id
WHERE reservations.time_from &gt;= CURDATE()
ORDER BY reservations.time_from;