the simplest way to do this is:
1> make a user with /bin/false as their user shell and the directory you want to allow access to as their home directory
useradd -d /path/to/the/directory/you/want/to/use -s /bin/false -g nogroup --no-user-group newftpusername
2> set a password and give it out to the people you want to allow access to
you could also have multiple different accounts all with read/write access to the same directory by making multiple users with the same group and the directory be group writeable
chown nobody /home/ftpdir
chgrp ftpgroup /home/ftpdir
chmod 775 /home/ftpdir
chmod g+s /home/ftpdir
chmod +t /home/ftpdir
useradd -d /home/ftpdir -g ftpgroup -N -s /bin/false user1
useradd -d /home/ftpdir -g ftpgroup -N -s /bin/false user2
then all users can have different passwords but upload to the same directory, also each user won't be able to delete the other's files (that's what the chmod +t does)
this is better because you can see who's uploaded what, so there's some measure of accountability.
best of luck!
if it's for someone you trust, that should work fine, but if you don't trust all the users, note that this type of user will have access to changing directory to / and therefore will be able to read almost all your system configuration files, etc. if you don't trust the users, make a public ftp server with a chrooted path and a public incoming directory, which should generally be world writeable and unreadable. you need to move stuff you accept to the server to a downloadable directory yourself or by an automated script (google on how to do this, zillions of guides out there). then again, remember public ftp servers with incoming directories have historically ended up getting used as warez sites a lot.