Combining two mostly identical rows in SQL

Question

I have a table that contains data like below:

Name	ID	Dept
Joe	1001	Accounting
Joe	1001	Marketing
Mary	1003	Administration
Mary	1009	Accounting

Each row is uniquely identified with a combo of Name and ID. I want the resulting table to combine rows that have same Name and ID and put their dept's together separated by a comma in alpha order. So the result would be:

Name	ID	Dept
Joe	1001	Accounting, Marketing
Mary	1003	Administration
Mary	1009	Accounting

I am not sure how to approach this. So far I have this, which doesn't really do what I need:

SELECT Name, ID, COUNT(*)
FROM employees
GROUP BY Name, ID

I know COUNT(*) is irrelevant here, but I am not sure what to do. Any help is appreciated! By the way, I am using PostgreSQL and I am new to the language.