Fore account creation, you can update this file to log:
/usr/lib/security/mkuser.sys
For example,
echo $(date)"\t"$(logname)"\t"$2"\t"$1 >> /my/log/path/mk_user.log
That will log the date, the user who executed the mkuser command (even if via smit), the account name and the home directory at the time of creation.
You can log further information if wanted.
Obviously, this will not trap user additions from editing /etc/passwd directly.
-glenn