WordPress as a CMS Membership website

I am involved with several WordPress support websites including this one (recently). I seem to get the question “Can WordPress be used as a CMS/Membership Website?”. I haven’t needed to set one up, so I always refer them to suggestion from other forums. I’ve decided that it’s only a matter of time until I will be required to implement this type of a WP website so I’ve decided to be Pro-Active and set up a development version.