need clarification regarding OpenId Philisophy

hi,

i would like to integrate openid authentication. now, my current setup is that the user has to go through my own registration & activation pages to get his profile active.

the way i understood eg railscasts etc to use openid and similar techniques, its up the app to decide whether the users can register and or do the login only via openid and similar.
now, in case of the login only, how do i map existing users to the ones who are loggin in via openid & similar (looked into openid-selector)?

maybe im overseeing something,

thx
tom