You can use the C# dynamic type to make things easier. This technique also makes re-factoring simpler as it does not rely on magic-strings.

JSON

The JSON string below is a simple response from an HTTP API call, and it defines two properties: Id and Name.

{"Id": 1, "Name": "biofractal"}

C#

Use JsonConvert.DeserializeObject<dynamic>() to deserialize this string into a dynamic type then simply access its properties in the usual way.

dynamic results = JsonConvert.DeserializeObject<dynamic>(json);
var id = results.Id;
var name= results.Name;

If you specify the type of the results variable as dynamic, instead of using the var keyword, then the property values will correctly deserialize, e.g. Id to an int and not a JValue (thanks to GFoley83 for the comment below).

Note: The NuGet link for the Newtonsoft assembly is http://nuget.org/packages/newtonsoft.json.

Package: You can also add the package with nuget live installer, with your project opened just do browse package and then just install it install, unistall, update, it will just be added to your project under Dependencies/NuGet

You can use this tool to create appropriate c# classes:

http://jsonclassgenerator.codeplex.com/

and when you will have classes created you can simply convert string to object:

    public static T ParseJsonObject<T>(string json) where T : class, new()
    {
        JObject jobject = JObject.Parse(json);
        return JsonConvert.DeserializeObject<T>(jobject.ToString());
    }

Here that classes: http://ge.tt/2KGtbPT/v/0?c

Just fix namespaces.

You have to deserialize existing json into list, add new person to existing users lists ( or maybe remove some) and after this to serialize the whole list again. Using append will never work, since json always must have only one root element, and you have to insert a new data inside of this root. But you are trying to add the second root and that makes json invalid.

    string json = string.Empty;
    using (StreamReader r = new StreamReader(jsonfilePath))
        json = r.ReadToEnd();

 List<Person> AllPeopleAdded = JsonConvert.DeserializeObject<List<Person>>(json);

List<Person> PeopleAddedThisTime = new List<Person>();
    
        
    //.....
    
    AllPeopleAdded.AddRange(PeopleAddedThisTime);

    using (StreamWriter file = File.CreateText(jsonfilePath))
    {
        JsonSerializer serializer = new JsonSerializer();
        serializer.Serialize(file, AllPeopleAdded);
    }

 // or  serialize JSON to a string and then write string to a file
File.WriteAllText(jsonfilePath, JsonConvert.SerializeObject(AllPeopleAdded));

and fix person class

public class Person
{
    public string mFirstName { get; private set; }
    public string mLastName { get; private set; }
    
    [JsonConstructor]
    public Person(string mFirstName, string mLastName)
    {
        this.mFirstName = mFirstName;
        this.mLastName = mLastName;
    }
    public string GetFullName()
    {
        return mFirstName + " " + mLastName;
    }
}