facebook connect question

hey

I am slightly confused as to how users are managed with facebook connect. Lets say they register through the main site, then the user info will be in the main site right? what about if users login in from the subsite using facebook connect, does the user get stored in the subsite or?

is this how it works?

user clicks on facebook connect on mainsite.com/wp-login and get registered on the main site
user clicks on facebook conect on subsite.mainsite.com/wp-login and user gets registered on the subsite.

do we have to use the register form of facebook to register a user, or can a user simply be registered by click on the facebook connct button and they grant access?