This seems to be a classical unix rights problem. Let's have a look on it. To avoid confusion I set user of `/media/NAS` to `root` and then check where is the problem:

    pi@raspberry ~$ sudo chown root /media/NAS
    pi@raspberry ~$ mkdir /media/NAS/folder-pi   # works
    pi@raspberry ~$ su -l pi2                    # login as pi2

    pi2@raspberry ~$ mkdir /media/NAS/folder-pi2   # works
    pi2@raspberry ~$ mkdir /media/NAS/folder-pi/folder-pi2
    mkdir: cannot create directory ‘/media/NAS/folder-pi/folder-pi2’: Permission denied

    pi2@raspberrypi:~ $ ls -la /media/NAS/
    total 16
    drwxrwx--x 4 root nasusers 4096 Mar 29 10:13 .
    drwxr-xr-x 3 root root     4096 Mar 29 08:38 ..
    drwxr-xr-x 2 pi   pi       4096 Mar 29 10:12 folder-pi
    drwxr-xr-x 2 pi2  pi2      4096 Mar 29 10:13 folder-pi2
    pi2@raspberrypi:~ $

Here we can see what has happend. Both users are in group `nasusers` so each can create folders in `/media/NAS/`. But the created folders get the user and group rights from the user, so pi2 cannot access folder from pi. `/media/NAS/folder-pi` is not in group `nasusers`. Also the group rights on the new folders are only read and execute `r-x`. We have to make sure that the group is inherited from the parent folder. For this we have the `sticky bit`. Also we have to make that the group gets `rwx` rights by default. For this is the `umask`. For it`s meaning look at the internet. Lets do it:

    pi2@raspberry ~$ rmdir /media/NAS/folder-pi2
    pi2@raspberry ~$ logout
    pi@raspberry ~$ rmdir /media/NAS/folder-pi

    pi@raspberry ~$ sudo chmod g+s /media/NAS/   # set sticky bit
    pi@raspberry ~$ ls -ld /media/NAS/           # it`s set on the group
    drwxrws--x 2 root nasusers 4096 Mar 29 10:40 /media/NAS/

    pi@raspberry ~$ umask 002   # set umask and check

    pi@raspberry ~$ mkdir /media/NAS/folder-pi
    pi@raspberry ~$ echo "hello world" >/media/NAS/folder-pi/file.pi
    pi@raspberry ~$ su -l pi2

    pi2@raspberry ~$ mkdir /media/NAS/folder-pi/folder-pi2
    pi2@raspberry ~$ cat /media/NAS/folder-pi/file.pi
    hello world
    pi2@raspberry ~$ ls -la /media/NAS/folder-pi/
    total 16
    drwxrwsr-x 3 pi   nasusers 4096 Mar 29 10:59 .
    drwxrws--x 3 root nasusers 4096 Mar 29 10:58 ..
    -rw-rw-r-- 1 pi   nasusers   12 Mar 29 10:59 file.pi
    drwxrwsr-x 2 pi2  nasusers 4096 Mar 29 10:59 folder-pi2
    pi2@raspberrypi:~ $

Now it's everything as it should be. For persistent settings of the `umask` you will find some information in `~/.profile`:

> # the default umask is set in /etc/profile; for setting the umask  
> # for ssh logins, install and configure the libpam-umask package.  
> # umask 022

Setting the default `umask` has a little disadvantage. It`s global and works also on the filesystem of your pi, so you have a little look at your group rights there.