You Can construct the JSON object for the list by using
System.Web.Script.Serialization.JavaScriptSerializer jSearializer =
new System.Web.Script.Serialization.JavaScriptSerializer();
string jsonData = jSearializer.Serialize(userList);
Now you can write jsonData to the file using
System.IO.File.AppendAllText(@"C:\Output.txt",jsonData);