How to hack a contact form?

I’ve just finished creating a members area for my clients. The only thing I want to change is the contact form.

Because the client logins in, I want to form to compromise of a text box to update me or a colleague.

But to make it work I need the contact form to know who is signed in.

So for example if Bob signs in with the username Bob and adds the message, Hi John please call me, then when the email goes to John (all contact forms are already set to go to the right people) I want the form to say the message was sent by Bob BUT I don’t want Bob to have to enter his name in a name section of a form.

With me?

Anyone know how I can do this?