You'll need to look at the classes in the
System.IO namespace to understand how to read text files.
You could use the
File.ReadAllLines method which will read all the lines from the file into a string array. You can then process the lines as required to create a list of users.
Better still would be to store the users as XML so you can use the native XML handling features of .NET to deal with that. If you structure the XML correctly you can read it directly into a DataSet so you don't even have to deal with any of the file parsing yourself. The DataSet class can load itself directly from XML. Then you can work with a table of users as if it came from a database.
-Peter