Well, John Lester pretty much summed it up in post #2: you need to get a copy of the file includes/functions_databuild.php on your local computer (either get it from the vb distribution, or download it via ftp from your server). If you download it from your server, make a copy of it on your computer so that you save the original. This way if something goes wrong with the changes, you should just be able to upload the unchanged file to fix it. If you're using the one from the vb distribution, make a copy of it before editing it.
Next you need to edit the file using a programming editor (or use notepad if you don't have anything else - don't use Word or wordpad or other word processing editor). Find this:
Code:
usergroupid NOT IN (3,4)
which is in two places: one is on a line by itself, the other is near the end of a long line. In both places you want to add 8 to the list, so it looks like this:
Code:
usergroupid NOT IN (3,4,8)
Then save it and use ftp to upload it to your server, in the includes directory, overwriting the existing file.