Won't work very well; I wouldn't recommend it. Here's why:
[*] Every file/directory in a UNIX system has exactly one owning user and group associated with it. You can't assign ownership directly to a group; there's always one user set as the owner.
[*] Only the user owning a file/directory (the user that created it) can ever perform certain operations, such as changing its permissions or group. You cannot "give away" ownership of a file to another user at all.
[*] There is no way to enforce what permissions other users apply to files/directories they create. The default permissions on files typically end up only allowing the owning user to modify them.
Trying to allow multiple users to modify files in the same directory without any sort of central management will typically end in tears (e.g, a mishmash of files, all with different owners and permissions, none of which can all be edited by a single user). As distasteful as it is from a security perspective, I'd strongly advise you to stick with a single FTP user. Alternatively, use a CMS such as WordPress, Joomla, or Drupal to manage your site contents.