Now that we have the CERN Open Hardware License on our registration page, I feel much more excited about inviting new people to join PLOTS -- so I just did so with a list of Kickstarter backers on the Balloon Mapping Kit project. Now any new user is asked to open source not only the content, but the hardware designs they publish on the site.
Anyone can invite others -- but PLOTS organizers on the "team" list (we're talking about maybe renaming that list) can do batch invitations, which is what I did in the above screenshot.
Then, when someone accepts your invite, you see a notification next time you use the site! Pretty cool, and of course it doesn't stop people from joining on their own -- you don't NEED an invite.