Mark, glad you like them.
Of course, in order to be as open as possible, there should be a way for people to 'apply for an invitation' - a bit like sending a covering letter explaining why they want to join and what they think they could offer to the community.
The site it would say something like "membership of the blog coop is by invitation only, but if you really want to join NOW, and are really serious about becoming an active member of our community, you can apply for an invitation by clicking here"
Existing members could set their profile to either accept applications or not. That way, if they didn't want to be bothered by invitation requests (for whatever reason), they wouldn't be.
Maybe if all members set their profile to 'no invite applications', then *no* invite applications would be accepted and the "apply for an invitation" bit wouldn't be displayed at all.
As soon as someone new is needed (perhaps to fulfill a specific task, or start a new project), whoever it is that wants help would change their profile and invitations would again be allowed.