If you need to let one user to a specific site, than you have to allow that by user id or IP address if you have static IPs.
I have rules that allow certain people to get to certain sites by user id. This is the best way I could think of to do it. If it is per user, then that means a rule set needs to be created for that user. You only need to create on rule for a user, then you can apply many sites within that rule.