not contained in either an aggregate function or the GROUPby

Msg 8120, Level 16, State 1, Line 1 Column 'Citys.City_Name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause. SELECT sum(Member_Info.IDCity), Citys.City_Name  FROM Member_Info INNER JOIN  Citys ON Member_Info.IDCity = Citys.IDCity  GROUP BY Member_Info.IDCity  And how can use where