Here you go. But I'm not sure this is what you want to do. You were talking about groups and I see nothing about that in the query.
DECLARE @Parameter varchar(12)
IF @Parameter = 'Credit'
BEGIN
SELECT
[Doc Number]
, [Customer Name]
, [Owner]
, [Rev Type1]
, [Mail Date]
, [Revenue Date]
, [Days Outstanding]
, [0-30 Days]
, [31-60 Days]
, [61-90 Days]
, [91-120 Days]
, [121-150 Days]
, [151 + Days]
, [Customer ID]
, [Total Outstanding]
, ( [0-30 Days] + [31-60 Days] + [61-90 Days] + [91-120 Days]
+ [121-150 Days] + [151 + Days] ) as [Total Outstanding2]
, [Reference]
FROM
#t1
WHERE
( [0-30 Days] + [31-60 Days] + [61-90 Days] + [91-120 Days]
+ [121-150 Days] + [151 + Days] ) < 0
END
ELSE
BEGIN
SELECT
[Doc Number]
, [Customer Name]
, [Owner]
, [Rev Type1]
, [Mail Date]
, [Revenue Date]
, [Days Outstanding]
, [0-30 Days]
, [31-60 Days]
, [61-90 Days]
, [91-120 Days]
, [121-150 Days]
, [151 + Days]
, [Customer ID]
, [Total Outstanding]
, ( [0-30 Days] + [31-60 Days] + [61-90 Days] + [91-120 Days]
+ [121-150 Days] + [151 + Days] ) as [Total Outstanding2]
, [Reference]
FROM
#t1
WHERE
( [0-30 Days] + [31-60 Days] + [61-90 Days] + [91-120 Days]
+ [121-150 Days] + [151 + Days] ) > 0
END