You definitely want a group for this. I would configure a group for FTP access. Put the users in a group that you want to use for FTP users, and then I suggest the following (assuming the FTP group is called FTP and the base data directory is /ftpdata - change these as needed):
chown -R nobody:ftp /ftpdata
find /ftpdata -type f -exec chmod 664 {} \;
find /ftpdata -type d -exec chmod 2775 {} \;
Here's what you're doing...
chown -R nobody:ftp /ftpdata
This sets the owner to nobody and the group to ftp for every file and directory below /ftpdata.
find /ftpdata -type f -exec chmod 664 {} \;
This command sets every file below /ftpdata to be mode 664, that is read-write for the owner and group, and read-only for everyone else.
find /ftpdata -type d -exec chmod 2775 {} \;
This command sets every directory below /ftpdata to 2775, that is read-write-and-execute for the owner and group, and read-execute for the world, plus any new files created in any of those directories will be owned by the ftp group.
The 2 in 2775 is "Set Group ID" - whenever a new file is created in a directory with that bit set, it makes the group of that file the same as the group that owns the directory. Without that, a user who's primary group is not the ftp group will create files that are owned by another group, and may not be accessible by other members of the group.