Note: this solution requires changing permissions for file creation for specific users and directories.
Setting the setgid permission bit on a directory that will be written into will keep all new files written in that directory assigned to the group of that directory (instead of to the group of the user that created them).
If the HTTP server's user creates files with group write permissions by default, then all files written into directories in the proper user group and with the setgit bit set will be fully accessibly by the users in that group.
Setting the setgid bit on
directoryname setting proper group ownership:
chmod g+s directoryname
chown :groupname directoryname
If group-write permission is not set by default, you'll need to set the umask of HTTP server user.