Don't forget that if you add a button to allow the user to join a mailing list then you will need a database to store the names.
This I think does not come with the contact form package. So you will need to do some coding yourself either by hand or using some WA extensions.
You could use the user registration solution pack to handle newsgroup registrations but thats more money.
What extensions if any do you have? Let us know and we can advise as to the best way forward.