Here is the painfully detailed version that assumes you know nothing :If you don't already have a members-only forum, see this IF documentation topic:
http://support.invisionfree.com/index.php?showtopic=20286. When you set up (or edit) the permissions, omit the guest and validating group from read/reply/post permissions.
Get a free chat room from
http://parachat.com. Make the box private to your site. Do that by naming the box something not guessable, and do not sign up for the network. You will have a choice of a pop-up code, or an embedded one. Choose the embedded code, highlight and right-click to copy it.
You will put the chatroom code in the Forum Rules section of a members-only forum.
To get to the forum rules, go to your Admin Control Panel
Find the Forum Control section in the list of links in the left side index.
Click on "Manage Forums"
You will see your forums listed in order. Find the forum you want to put the chatroom in. Then follow right. You will see links to "Settings" and "Permissions". The next link is a yellow triangle icon. That is the forum rules. Click on it.
That will take you to the Forum Rules edit screen.
In the first box (Display Method), choose "show link only"
In the second box (Rules Title) put something like "Enter Private ChatRoom"
In the third box (Rules Text) is where you will paste your code that you have copied from the ParaChat site. (It should be the code that is for the embedded chatroom.)
When you have pasted the code there, go to the bottom of the screen and click on "Edit Forum Rules".
Now, go back to your board and enter the members-only forum. you will see the little triangle icon above your forum with the title that you named it. Click on that. It will open up ("pop") your chatroom.
Now, look in your addressbar....highlight and copy the URL that you see there. You will need that address to make a link in your members-only forum description.
Paste the link into this code: