SQL Query Syntax

I'm trying to use the GROUP BY statement but I'm getting an error message when I try to execute my query. The error states: Column 'dbo.jomast.fsono' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Not sure I understand what I'm doing wrong. Can anyone shed some light on this?

Here's the query:

SELECT        J2.fjobno AS JobOrderNo, J2.fsono + LTRIM(RTRIM(J2.fkey)) AS SOKey, I.fcpro_name AS WorkCenterName
FROM            dbo.jodrtg AS J1 INNER JOIN
                         dbo.inwork AS I ON J1.fpro_id = I.fcpro_id INNER JOIN
                         dbo.jomast AS J2 ON J1.fjobno = J2.fjobno
WHERE        (I.fcpro_name LIKE '%-G%')
GROUP BY J2.fjobno

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