MySQL: combining multiple values after a join into one result column

Stuart's answer is fine. This is just to show the working version of your approach:

SELECT p.`id`, p.`title`, a.`fullname`
FROM `publications` p LEFT JOIN
      (SELECT publication_id, GROUP_CONCAT(a.`fullname` separator ', ')
       FROM `authors` a
       GROUP BY publication_id
      ) a
--------^
      ON a.`publication_id` = p.`id`;

The error you got was because the a was missing after the subquery. The subquery also needed to be fixed, to include publication_id in the select and a group by clause.

You need to group by all of the non-aggregated columns in the SELECT (and explicitly, not group by the author id, because author is part of the GROUP_CONCAT):

SELECT p.`id`, p.`title`, GROUP_CONCAT(a.`fullname` separator ', ')
from `publications` p 
LEFT JOIN `authors` a on a.`publication_id` = p.`id` 
GROUP BY p.`id`, p.`title`;