Take a look at this article:

Generating n-Tuples with SQL[/url]

Then look into the discussion thread for a slight performance improvement on the approach:

http://www.sqlservercentral.com/Forums/Topic1301485-3122-5.aspx

Coding up the improved approach with your sample data yields this:

WITH SampleData (Usr, Grp) AS

(

SELECT 'SmithA','GrpA'

UNION ALL SELECT 'SmithA','GrpB'

UNION ALL SELECT 'MouseM','GrpD'

UNION ALL SELECT 'MouseM','GrpB'

UNION ALL SELECT 'MouseM','GrpC'

),

UNIQUEnTuples (n, Usr, Tuples, ID) AS

(

SELECT 1, Usr, CAST(Grp AS VARCHAR(8000)), Grp

FROM SampleData

UNION ALL

SELECT 1 + n.n, n.Usr, t.Grp + ',' + n.Tuples, Grp

FROM UNIQUEnTuples n

CROSS APPLY

(

SELECT Grp

FROM SampleData t

WHERE t.Grp < n.ID AND t.Usr = n.Usr

) t

-- WHERE n <= 5

)

SELECT *

FROM UNIQUEnTuples

ORDER BY Usr, n;

This has the potential to run like a pig if you really do have 32 groups possible for a Usr. You might be able to improve the performance by running the anchor leg of the query first to a Temp table and then using a WHILE loop to implement the recursive leg.

If you're not sure how to do this, let me know.

Edit: I added the commented WHERE clause, because you might want to run it with that WHERE clause uncommented the first time you try with your 32 groups, just to see how long it takes.