What is the best way to handle a join / group_concat query in MySQL
Simplicity is beautify. Multiple JOINs are allowed. There is no need to the JOINs to be LEFT JOINS as you want data that maches always.
SELECT tasks.task, GROUP_CONCAT( users.name SEPARATOR ',' ) AS names,
FROM assignees
JOIN tasks ON assignees.task_id = tasks.id
JOIN users ON assignees.user_id = users.id
WHERE tasks.completed IS NULL
GROUP BY assignees.task_id