FTP makes this difficult, but if your FTP client can use SSH instead, then it's possible for users to share a common directory. You just need to make the directory group writable.
Provided that you're the one who created the users, this will work. One aspect of this is that others can not erase another user's file, which may be a good thing. You, as the master use, also can not erase others' files, but if you're a cooperative group, file maintenance shouldn't be a problem.
FTP won't work because a user can not leave their home directory, whereas using SSH (SCP), you can go to another user's home directory provided its permissions allow you to.