Hi. You might also check out our Dynamic Registration module, many users use the module with Private Registration set.

In this case do you mean when they click on 'Register' they are prompted for the sign in or simply after registration? If they are prompted for login you can just change that page to be viewable to all users. If after registration you might also consider displaying a message to them or sending them in email that registration needs to be authorized before they can continue.  I would probably ignore the redirection after registration (if you are using your own custom coded registration module?).

Again though, our module handles all of these along with many other features so you might want to consider it depending on the full specs and budget etc...

-Chad