Edit your /etc/ftpaccess file
This is a default ftpaccess file
class all real,guest,anonymous *
email root@localhost
loginfails 5
readme README* login
readme README* cwd=*
message /welcome.msg login
message .message cwd=*
compress yes all
tar yes all
chmod no guest,anonymous
delete no guest,anonymous
overwrite no guest,anonymous
rename no guest,anonymous
log transfers anonymous,real inbound,outbound
shutdown /etc/shutmsg
passwd-check rfc822 warn
You want to change it so it looks like this
class all real,guest,anonymous *
email root@localhost
loginfails 5
readme README* login
readme README* cwd=*
message /welcome.msg login
message .message cwd=*
compress yes all
tar yes all
chmod no anonymous
delete no anonymous
overwrite no anonymous
rename no anonymous
log transfers guest,anonymous,real inbound,outbound
shutdown /etc/shutmsg
guestgroup users
passwd-check rfc822 warn
You are defining group "users" as a guest group with "guestgroup users"
By removing "guest" from the chmod, delete, overwrite, and rename lines your giving them the ability to fully manipulate their files.
By adding "guest" to the "log transfers" line your enabling logging of transfers to the system log for the "guest" group.