GROUPing only those rows that do not contain a NULL field?

Hi

I wonder if anyone can help me with a SELECT / GROUP BY problem I'm having.
I'm trying to refine a query so that my GROUP BY clause only aggregates rows
that have a non-NULL value in one of the fields, leaving other rows
'ungrouped'.

An example table, resulting from a LEFT JOIN and subselect on three tables,
might be (before grouping) :

SELECT user_group_id, user_id, topic_id FROM user_groups NATURAL JOIN users
LEFT JOIN
(SELECT user_id, topic_id FROM topic_participants WHERE topic_id = 567) AS a
USING (user_id)

user_group_id | user_id | topic_id
-----------------------------------------
1 101 NULL
1 102 567
1 103 567
2 101 NULL
2 106 567
3 101 NULL
3 104 567
3 102 567
4 103 567
4 104 567
5 105 NULL
6 103 567
6 104 567

(topic id is either a single value, or NULL if the user is not part of the
topic)

For UI reasons I would like this result collapsed thus:

user_group_id | user_id | topic_id
-----------------------------------------
1 101 NULL
1 102 567
1 103 567
2 101 NULL
2 106 567
3 101 NULL
3 104 567
5 105 NULL

Note that for all rows where topic_id IS NOT NULL, only one row per user_id
is returned (user_group_id is not relevant for these rows).

Can this aggregation be achieved with a DISTINCT ON / GROUP BY clause? (Or
anything else). I haven't managed to find anything that leaves the
NULL-field rows unscathed so far.

Thanks in advance for any help!

Simon K