Using SELECT result in another SELECT

NewScores is an alias to Scores table – it looks like you can combine the queries as follows:

SELECT 
    ROW_NUMBER() OVER( ORDER BY NETT) AS Rank, 
    Name, 
    FlagImg, 
    Nett, 
    Rounds 
FROM (
    SELECT 
        Members.FirstName + ' ' + Members.LastName AS Name, 
        CASE 
            WHEN MenuCountry.ImgURL IS NULL THEN 
                '~/images/flags/ismygolf.png' 
            ELSE 
                MenuCountry.ImgURL 
        END AS FlagImg, 
        AVG(CAST(NewScores.NetScore AS DECIMAL(18, 4))) AS Nett, 
        COUNT(Score.ScoreID) AS Rounds 
    FROM 
        Members 
        INNER JOIN 
        Score NewScores
            ON Members.MemberID = NewScores.MemberID 
        LEFT OUTER JOIN MenuCountry 
            ON Members.Country = MenuCountry.ID 
    WHERE 
        Members.Status = 1 
        AND NewScores.InsertedDate >= DATEADD(mm, -3, GETDATE())
    GROUP BY 
        Members.FirstName + ' ' + Members.LastName, 
        MenuCountry.ImgURL
    ) AS Dertbl 
ORDER BY;