Are there any articles out there that weigh the pros/cons of invitation-only websites? I'm curious as to whether it would actually be advantageous to be invitation-only rather than open-to-all. By making your site invitation-only, you're simultaneously increasing demand for membership (people want what they can't have), and boosting advertising by word of mouth. Take gmail for example, would they have been as explosively successful if they hadn't made membership invitation-only? Thoughts?