Thanks for your help, and I've learned a new thing in vBulletin
I'm going to test something and I changed the Unregistered ID in usergroup table to 12.
It seems my test worked well, not perfectly as I expected, but it's OK
There is more than one table that calls the usergroup ID.. :erm:
FYI.. there are 85 matches for "usergroup" in the vbulletin schema, which you can find in your install folder..
You will break the site if you changed one usergroup ID as you said..