If you use SAM or SMH, you can track useradd activities. However, there is no specific log for the useradd command.
The usual workaround for this is to rename useradd (something like useradd.orig) and then create a script which creates the log entry you need (a custom log file or into syslog.log) and then does an exec to replace the script execution with the real useradd. Another common use of a script wrapper like this is for the find command (to trap senseless find / commands that can impact the entire system).
