It is possible to connect geoNetwork to different authentication systems like LDAP or CAS, which is the recommended way of sharing users between different apps: http://geonetwork-opensource.org/manuals/trunk/eng/users/admin/authentication/index.html
But probably you are talking about a customized database with users and groups. There is no generic way to do this (and probably it is not recommended), but you can try to use views on your database to try to mimic geoNetwork user tables using your own data: http://www.postgresql.org/docs/9.2/static/sql-createview.html
If you use views, remember that you will have to create special triggers if you want to be able to administer users from the geoNetwork UI.