LDAP is probably not the easiest data base to manage. Every time you require adding, editing or removing entries you ought to create a LDIF file. Yet the format is quite simple it takes time to write and you have to learn and remember its syntax.
There are many GUI available, both free and commercial. Sun also provides a LDAP editing tool as part of the Sun Java Web Console. However these tools are for general LDAP editing and not just for user's management.
Some months ago I wrote a Web GUI for user's management for me to use and I know decided to make it public. It was not meant to be a shared tool so they're are quite a few hardcoded variables. I hope you find it useful nevertheless and if you require custom modifications feel free to contact me and we can talk about it.
