How to recover the built in Directory Service data?
There is a daemon that backups the directory /private/var/db/dslocal nightly. If the backup hasn't occurred since the removal of the user then the info could be restored. The archive is /private/var/db/dslocal-backup.xar.

An alternate solution for "system" users and groups:

Rebuild the corresponding user and group plist's from /private/etc/passwd and /private/etc/group.