How to achieve Assign Unique Value

Hi,

I have a task where I need to assign a new value depending If a Group number falls in particular M area.

For an example : In Database I have:

SELECT DISTINCT 
M,[GroupNumber]
FROM    [COM_Util].[dbo].[Milliman_Max]
WHERE MM=608
ORDER BY  [GroupNumber]

WHAT I NEED is 

Please Kindly help me.