Multi-User Login (.bin File)

I'm currently working on a program to store files and information.
I'm wanting the program to be able to have multi-user logins i.e. there are numerous combinations of usernames and Passwords.

I've created a program that will create new accounts and save them in a .bin file(in the form Username Password).

However, now i need to use the user accounts to grant access to a form.
Any tips on how to go about creating this? If you could give a quick example it would be much appreciated.

thankyou.

Follow the CodeBank link in my signature and you'll find a submission dedicated to a WinForms Logon.