A private group means the name of the group is not visible to the public. Anyone can join, but they must know the group code. You can join a private group by being invited into a group by someone already in it or you can register on your own and enter the group code which might look something like ACME1234. (It's auto-generated when you create the group, usiing the first four chars of your group name, then randomizing four digits)
Let's assume you have been given a group code to join. Here are the four steps required:
1. Create an account -- (it's free)
2. After you login ... Click Select Groups icon from the Control Panel
3. Click Join a Group
4. Select Private, then enter the group code
In some cases, you may have received a post card with the group code displayed in the corner... or it may have just showed up in some random email from one of your friends or posted on a website. However you received it doesn't really matter... all you need to know is that it goes into the Private field.
