Given a group 'webusers' which contains your five users:
# mkdir /some/path/shared-folder
# chown root:webusers /some/path/shared-folder
# chmod 2770 /some/path/shared-folder
Now everything created within shared-folder will have group ownership of 'webusers'. You may additionally need to do a little umask magic to get things working just right.
$ touch foo
$ ls -l foo
-rw-r--r-- 1 e e 0 May 22 12:14 foo
$ umask 0006
$ touch bar
$ ls -l bar
-rw-rw---- 1 e e 0 May 22 12:14 bar
Permanent umask settings changes can be placed in each users' ~/.bashrc file.