Just create a forum, you can get code off the net if you don't already have your own.
In your database where all the private members profiles data is stored, in the table where profiles are stored add a field for activationCode, or the like. This will be some type of confirming setting which only the 500 members from your private list will be given access to.
When a private member creates their profile to become a member of your private forum (just like a normal forum that we are discussing this in), add a form field such as "Private Member Code". Since ONLY the private members will have the single code you created, that is pretty well private. In your ASP scripting that the new member form gets sent to for processing, make sure that the "Private Member Code" they entered matches the code you assigned, that is to say:
Code:
If Request.Form("txtPrivateCode") = "PrivateMemberCode" Then
Response.write "Okay"
ELSE
Resposne.write "Not Okay"
End If
That should give you some idea. How you distribute the Private Member code is up to you. There are multiple variations on this idea but your initial question was Ueber-vague.
Cheers,
Christian