Getting an average from a C# array

1. You have a numPlayer variables that stands for a number of entered players.
   Just use it.

Replace

        avgScore = total / score.Length;

with

        avgScore = total / numPlayer;

2. Your code has some very strange points.

For example, you call CalculateAverageScore(score, ref numPlayer); in a Main(). However, you are not using a return value. This method is properly called in a DisplayBelowAverage method. In general, it looks wrong - refs, static-sized array with dynamic number of values, non-format console writeline etc.

Just for you information. Read this once. Maybe some code lines will help you. Maybe, you won't find something new, unknown or interesting.
This is how I would solve this problem:

public class Program
{   
    private const string InputTerminationString = "Q";

    public static void Main()
    {
        List<Player> players = new List<Player>(); // p. 1, 4

        while (true)
        {
            Console.Write("Enter player name ({0} to quit): ", InputTerminationString);
            string name = Console.ReadLine();
            if (name == InputTerminationString) break; // p. 2

            Console.Write("Enter score for {0}: ", name); // p. 3
            int score = int.Parse(Console.ReadLine());

            players.Add(new Player { Name = name, Score = score }); 
        }

        Console.WriteLine("Name           Score");
        players.ForEach(x => Console.WriteLine("{0, -16} {1, 8}", x.Name, x.Score)); // p. 5

        double average = players.Average(x => x.Score); // p. 6
        Console.WriteLine("Average score: {0:F2}", average); // p. 3

        Console.WriteLine("Players who scored below average");
        Console.WriteLine("Name           Score");
        players
            .Where(x => x.Score < average) // p. 7
            .ToList()
            .ForEach(x => Console.WriteLine("{0, -16} {1, 8}", x.Name, x.Score)); // p. 5           
    }
}

public class Player
{
    public string Name { get; set; }
    public int Score { get; set; }  
}

Now, step by step:

1. Use Player class. It is pretty inconvenient to pass two independent arrays of names and scores. Moreover, it is not safe and proper in general. A name and a score of a player are properties if a single player and should be stored together either in struct or class.

2. Use constants. If you need to change 'Q' termination string to 'Exit', you will be able to do this in a second without looking through the code.

3. Use formatted Console.WriteLine. It works liks String.Format and you don't need to concatenate strings.
   Some info here: https://msdn.microsoft.com/en-us/library/828t9b9h(v=vs.110).aspx

4. Use dynamic collection List. Array is good for storing and rapid accessing to range of values of known length. Since a user enters values one by one, you never know if he will enter 0, 1 or 90000 values. List collection will help you with it.
   Some info here: http://www.dotnetperls.com/list

5. You can use ForEach method which executes the given code for every item in a collection. Using ForEach with lambda-expressions makes code shorter and more readable.
   Some info here: https://msdn.microsoft.com/en-us/library/bwabdf9z(v=vs.110).aspx

6. Use Average function which calculates the average of a collection. In case of int[] you can use arr.Average(). However, in case of Class object, you need to describe the logic of calculation of average value using lambda-expression.
   Some info here: http://www.dotnetperls.com/average

7. LINQ Where expression lets you filter your collection.
   Some info here: http://www.dotnetperls.com/where

Instead of writing CalculateAverageScore the you can make use of LINQ's Average method:

double avgScore = score.Average();
Console.WriteLine("Average Score:" + avgScore);

Forthermore it is better not to mix calculating with I/O, so:

static double CalculateAverageScore(int[] score) {
     return score.Average();
}

Or since you use an array, use the sum:

static double CalculateAverageScore(int[] score, ref int numPlayer) {
     return (double) score.Sum()/numPlayer;
}

and do the printing in the Main:

static void Main(string[] args) {
    //Variables
    string[] player = new string[100];
    int[] score = new int[100];
    int numPlayer = 0;

    InputData(player, score, ref numPlayer);
    DisplayPlayerData(player, score, numPlayer);
    double avgScore = CalculateAverageScore(score, ref numPlayer);
    Console.WriteLine("Average Score:" + avgScore);
    DisplayBelowAverage(player, score, numPlayer);
    Console.ReadLine();
}

online Ideone demo.

You furthermore better modify the array to a List. Here there is also no reason to use the ref keyword in all these methods, nor to make them static.