Changing Authenticating DC’s

Site A and Site B are connected via T-1. Site A has had two Domain Controllers. Site B had no DC’s until today. We add the DC, but users on Site B subnet are still authenticating to Site A DC’s. Why? What can I do to force Site B Users to authenticate to SIte B DC?
100% Win2k Domain. Perhaps I have not setup my Site correctly. Perhaps there is a Group Policy that will force Users/Groups/Locations to use a certain DC. All help appreciated!