The issue of credit unions making it easy for anyone to join is reviewed by Ron Lieber in this New York Times column
It includes the history of how Pentagon Federal Credit Union changed to its open membership via an easy-to-join association. Also, it reviews the issue of phantom associations that some credit unions use to allow anyone to join.
Should consumers take advantage of this? Here's what Ron Lieber says at the end of the column:
I’m not sure who is right here, but if you’re a consumer it doesn’t really matter. [...] And until the industry regulator stops allowing many of the biggest credit unions to offer services to anyone who shows up or logs in, you’d be foolish not to check out a few the next time you need financial services.