How to get list of values in GROUP_BY clause

id   data
--   ----
1    1, 2, 3
2    4, 5
3    6, 4

CREATE FUNCTION [dbo].[GetDataForID]
(
    @ID int
)
RETURNS varchar(max)
AS
BEGIN
    declare @output varchar(max)
    select @output = COALESCE(@output + ', ', '') + data
    from table
    where ID = @ID

    return @output
END
GO

CREATE FUNCTION [dbo].[GetDataForID]
(
    @ID int
)
RETURNS varchar(max)
AS
BEGIN
    declare @output varchar(max)
    select @output = COALESCE(@output + ', ', '') + data
    from table
    where ID = @ID

    return @output
END
GO

SELECT ID, dbo.GetDataForID(ID) as Data
FROM Table
GROUP BY ID

SELECT id,
       LIST(data)
FROM   table
GROUP BY id

SELECT id, LIST(data||', ')
FROM yourtable
GROUP BY id