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