Assinging author to posts when author is a registered user

I want users that have already registered to be able to use a form to submit posts without having to log on every time they need to submit something.
I've tried using the register user plugin, but it creates a new user.

So are any of the below possible:
1. user enters credentials in form along with the post information and the form automatically logs them in and posts the article under their name.

2. user enters credentials in form and the the information is submitted with the name set to the user's name without having to log them in?

Thanks