Have you tried Yammer.com?
Use it for free if all your users have the same ‘domain’ on the end of their email. For example, if your entire office has teamexpansion.org email addresses, ask them to log onto Yammer and you’re in business! Instant Intranet! Learn more at the site. No charge as long as you don’t mind allowing things to be a bit “loosey-goosey” on administration. The network will be encrypted, mind you, so outsiders won’t be able to see or learn about your personnel nor about an of your sensitive fields. And along the way, you’ll pick up free calendaring, free discussion threading, free Facebook-like status updates, and much much more.
We are just starting to use Yammer as a safe place to talk about lots of stuff. Integration with our (secure) intranet was pretty easy. The premium version is well worth paying for as the extra features give you a lot more control.