If that's the case, I think it should be communicated as a security precaution rather than a"private members club." Certain voting behaviours are already beginning to foster a feeling of "insiders" and "outsiders"... this step could exacerbate that.
Having someone that can vouch for a new member may work as it should raise the quality of content. However even that won't legislate against hackers/lax passwods etc.
that would not be a wise on the branding and marketing side. Message it has invitation only and then move forward as invitation only too. They need to control the growth of it and putting a dampener is wise at this point. It's not a lie to say that it's under invitation only when it is. I use "private membership" term to illustrate a phenomenon that people will experience by being invited.