I suspect “welcome, please register” popup is working quite well with my forum and I started to get more registrations from random readers!
I just thought it would be interested and useful to track which website means were used by a user for registration - Login button, Signup button, “welcome, please register” popup, by-email-invitation etc. This way we would also be able to query database for statistics, learn what method works best for a particular community and focus efforts on that particular registration method.