How do I give multiple users access to a Windows NTFS partition?

Mounting with specific permissions

There is. Put both users in a group together. Then use that group's to set the gid= option and and mount your disk with the permissions you want it to have. You can control permissions with umask=, fmask= and dmask= options in the mount command. Here is an example:

mount -O dmask=007,fmask=117,gid=46,uid=1000 /dev/sdc1 /media/winhdd

These options can also be used in /etc/fstab like this: UUID=7258CB9858CB598D /media/win ntfs rw,auto,user,exec,nls=utf8,dmask=007,fmask=117,gid=46,uid=1000 0 2.

Avoiding fstab - automounting a specific disk with udev

I imagine you want to avoid fstab because the disk it not always connected on boot. If you wish to avoid fstab use a udev rule. Udev can automatically mount (and prevent Ubuntu from automounting) when the disk is connected. A rule for that may look like this:

# In /etc/udev/rules.d/70-usb-winhdd-mount.rules

KERNEL=="sd?[0-9]", ATTR{removable}=="1", ATTRS{serial}=="UA04FLGC", ACTION=="add", RUN+="mount -O dmask=007,fmask=117,gid=46,uid=1000 /dev/%k /media/winhdd"

You will need to identify the disk by it's properties. In my example I did this by ATTRS{serial}=="UA04FLGC". I described how to do this in another question. The naming and location of the rules files is described there too.

Give all removable disk those properties

If you want to do this with all removable disk - not just that particular one, simply leave out the serial part and be sure to assign a unique mountpoint (counting them, creating from serial, %-symbol of udev or other method). Also as stated elsewhere ENV{mount_options}="$env{mount_options},dmask=007,fmask=117,gid=46,uid=1000" could be an option for mounting all removable disks with certain permissions/onwership.

Useful information you might already know:

Creating a group and adding users

Those are the shell commands to create a group named 'winhdd' and add a user named 'confus' to that group:

groupadd winhdd
usermod -a -G winhdd confus

Finding out the group id

You can find the group ids of all the groups on your system in a file called /etc/group. The entries there look like winhdd:x:4:confus,narur,joe where 'confus', 'narur' and 'joe' are the user names belonging to that group, 'adm' is the group's name and '4' the group id, you're looking for. In /etc/groups you can also check if adding the users to your group was successful. The command getent group winhdd would show the line with information about the group. The id command gives you your group id, user id and the names and ids of all groups you belong to.

Setting umask-like options

You'll want to set dmask= to a sensible value allowing you at least to execute (=access) directories. fmask= is the option for file permissions.

dmask=007,fmask=117,gid=46,uid=1000 allows the owner that is user number 1000 (first digit in the masks) and the the members of group number 46 (second digit in the mask) to read and write and files on the disk. Other users have no rights to do anything (hence third digit in the masks - the 7). Here are the mask values:

7 – no permissions     6 – execute only    5 – write only
4 – write and execute  3 – read only       2 – read and execute
1 – read and write     0 – read, write and execute