Additional field in register.php

Hi

I would like to add an extra field in register.php to select by new user during registration process.

My first thought was at the end of registration form add text saying
by registering on web site you automatically become saved to our mailing list.
I want to inform users about news on web site.

Second thought was add some checkbox an if user don't select it he could not be able to register.

I would be grateful if someone could provide a solution to the first and second case.

regards