Once you have your LiveJournal server tuned and tweaked, you are probably going to want to create some users and communities.
Creating users is relatively simple, and depends on which options you are using in your etc/ljconfig.pl.
If you are not using this file for your setup, then the default options defined in cgi-bin/ljdefaults.pl will be assumed.
If your service is live, simply visit /create.bml. With this page, you can create as many users as you like.
If you are using LDAP, you should instead create accounts in your LDAP external authorization source.