All the people registered for the forum are star ranked members of the clan, and I personally pestered them to join in. Since star ranked members kind of need to be a part of the offsite to keep up to date.
EDIT: There was also the issue that people don't know this forum host as well, I've used DarkBB which is exactly the same setup, so they are probably affiliates. But I chose Zetaboards because its very well know and used by many people so people would be less conflicted to joining the Offsite. The dowside though is the layout is much harder to modify than these forums because everything is CSS and HTML.