KarinUlli on "Redirecting login to another website"

Hi!
I'm making a site for a client. He has already a site with an user system, where the users log in to a "back end" on a different web-page. So say the site's adress is http://www.companyname.com, and there you can find a login-form wich sends you to http://www.theservicethecompanyoffers.com :) I want this users to be able to log in from the new website I'm creating, but I'm a little lost, I think.
After some research I think I need to first import the users to the wp database, and then redirect the login url to http://www.theservicethecompanyoffers.com. Is this right? In that case, how do I redirect the users there?
And by the way, I'm using a child theme.