Surprisingly, there was no way to start the server automatically during boot-up. Considering that Directory Server is a Red Hat product, I expected it to include an rc script allowing for this, in addition to easy start, stop, restart and status functions.
I created three clones of my original RHEL 4 virtual machine and set up Directory Server on each to test the multimaster replication scenario detailed in the Administration Guide provided. Each server acted as a "consumer" and a "supplier" of data to the others in the replication process, meaning directory information modified on one server was quickly replicated to the others. When the same information is modified on multiple servers, Directory Server resolves the conflict by treating the most recent change as the valid one.
To add users and groups, I tested both the Console and ldapmodify, a command-line tool that is part of the OpenLDAP tools package I installed. It was easy to modify directory content in the Console GUI, which meant I didn't have to become overly familiar with ldapmodify.