Login but not register w/ FB Connect

We have a buddypress site that is protected with the sign-up code plugin and has zero content available until you are logged in (using the membership plugin). I would like to use Ulitimate Facebook to allow logins using FB connect, but want to limit it only to those who have already signed up.

Am scratching my brain trying to figure out how to do this using the Ultimate FB plugin...

Thanks for the help!