If users access directly the WDTV Live media player, then is is up to what the player can do.
If the player is connected to a computer and the users has to access the computer in order to use the player then you can use the Win 7 Permission & Security to control the Access.
Permission and Security issues with Vista/Win7, check the following settings.
All users that are allowed to share need to have account on
all the computers that they are allowed to connect to.
Everyone is an account, it means a group of all of the users that already have an account and been established as users.
Using the
Everyone feature saves the need of configuring permission to each of the
established users, it does not mean
Everyone that feel that they would like to login.
Users that do not have an account on the computer are Not part of the Everyone Group.
If security on the LAN is
Not needed and users are Not established, then switching On the
Guest account provides semi-open configuration.
http://windows.microsoft.com/en-US/windows7/Turn-the-guest-account-on-or-off
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Point to the a Folder that meant to to be share. Right click and choose properties.
In the properties
Click on the Security tab shown in the pic bellow to the right) and check that the users and their permission (shown in the pic bellow center and left) are correctly configured. Then do the same to the Permission tab.
This screen shot is from Win 7, Vista's menus are similar.
http://www.ezlan.net/Win7/Permission-Security.jpg
In both the Security panel, and the Permission panel you have to highlight each User/Group and examine that the Permission Controls are checked correctly.
When everything is OK, Reboot the Network (Router, and computer).
* Note . The Groups and Users shown in the screen-shoot are just an example. Your list will look the way your system is configured.