FTP :

The /etc/ftpusers file contains a list of local user names that the ftpd
server does not allow remote File Transfer Protocol (FTP) clients to use
(i.e. list of user names not allowed FTP access). The format of the ftpusers
file is a simple list of user names that also appear in the /etc/passwd
file.

So if it is missing means that a FTP client can logon to the FTP server as
any user defined in the /etc/passwd file on the FTP server.

For security reasons you should have the following entries in the
/etc/ftpusers file:
root
daemon
bin
sys
adm
lp
uccp
nuucp
listen
nobody
noaccess
nobody4
........ Any other users not allowed fo ftp to this server.

Use following commands to maintain /etc/ftpuser (or use smit ruser): -
ruser -a -f <user> Adds entry into /etc/ftpusers file
ruser -d -f <user> Deletes entry in /etc/ftpusers file
ruser -s -F Shows all entries in /etc/ftpusers file
ruser -X -F Deletes all entries in /etc/ftpusers file