You can add users to your DB manually as admin on /acs-admin/users, or new users can register themselves by visiting /register/, which they are being redirected to automatically if necessary, e.g. before posting to a forum.
In case you want to do this in your own code, start by an api search for procs that contain 'user': http://openacs.org/api-doc/proc-search?query_string=user&search_type=Search&name_weight=5¶m_weight=3&doc_weight=2
. Seems that ad_user_new could do it. This is for openacs 4.6, you might want to search /api-doc/ on your own installation.