CLass diagram

I am not sure you actuallyu want a class diagram. This will show the classes involved in a domain, however you are asking about a use case. This would be best illistrated using either object diagrams, to show the state before and after the login, or a sequence diagram to show the dynamic flow of messages. I doubt if anyone will draw this for you, but if you want a simple [and easy to use] tool, try umlet.

We're not going to email you anything, but you'll get plenty of spam putting your address on a public forum like this.  If you can't do a job assigned to you, you'd be better using the job board to offer to pay someone to do it for you.  These forums are for people trying to do their own work and needing specific help.