My network has been running for over 10 years and I switched from 2.0 to 3.0 a few months ago.
I've always been set to join by INVITATION ONLY and I email a personal link to each new member.
However, I noticed that a few new members had joined without the link and when I checked the site there was a SIGN UP button.
The PRIVACY setting INVITE ONLY was de-selecetd.
I'm 99% certain that I didn't change that and that I checked it when moved to 3.0 - interesting to know if anyone else has seen similar?