How can I let users choose their own usernames and passwords?

My website is in the beta stage, and we want users to receive either a coupon code or an invitation code via e-mail and enter it to obtain a one-week pass to review the site. Is a coupon code or an invitation code more appropriate?
I understand coupons have a fixed expiration, so I'm leaning toward invitations.
How can I let invited users choose their own temporary username or password with a form so that I don't have to do it manually (that is, I want them to be able to immediately choose login/password info, and not have to wait for me to set it up).