How to concatenate data from one field, in a comma-delimited list, in a many-to-many relationship in MySQL?

I have a many-to-many relationship between People and Departments since one person can be in many departments.

People          Departments
------          -----------
pID  pName      deptID   deptName
1    James      1        Engineering
2    Mary       2        Research
3    Paul       3        Marketing
                4        Communications

People_Departments
------------------
pID   deptID
1     1
1     2
2     2
2     4
3     1
3     2
3     3

What I want is this:

pName  deptName
James  Engineering, Research
Mary   Research, Communication
Paul   Engineering, Research, Marketing

If I do plain LEFT JOINs on the tables using the SQL below, I will get several rows related to one person:

SELECT people.pName,
       departments.deptName
FROM people
LEFT JOIN people_departments ON people.pID=people_departments.pID
LEFT JOIN departments ON people_departments.deptID=departments.deptID

I have tried various combinations of GROUP_CONCAT but without luck.

Any ideas to share?