Username/passowrd generation when sign up for email newsletter [on hold]

I have what may a newbie question but I want to set up an email campaign for a client. We want to offer free access to a forum on his website so this means people who sign up (give their names and email addresses) will need usernames and passwords to access the forum.

I don't know the best way to accomplish this. I want users who sign up to get a welcome email that gives them immediate access to the forum as a gift for signing up.

When I have purchased things online I usually get a username and password emailed to me when I receive the welcome email but I'm not sure how to accomplish this. Can someone give me some advice or point me in the right direction? Thanks!