Why do people join groups? Why do people buy stuff? According to Darren of ProBlogger because other people do. He calls it social decision making and says it has improved his affiliate marketing sales. There is a book about a guy who put an ad in a newspaper and said just "Join me!" and people did. I've not read it yet, but I'm going to keep a look out for it.
Maybe we should put an ad on Craigslist and say "Join us!"