PGP Command Line is licensed per machine. The license information is stored in the preferences file, but by default the preferences file (and keyring files) are stored in directory that is relative to the current user.
You can see this by running "pgp --version --verbose" as both the original user (who licensed PGP Command Line) and as another user. Look at the "File information" section of the output.
The solution is to use a common (shared) "home directory". This is a home directory for PGP Command Line, not a home directory for Windows. Use the "--home-dir" option when invoking PGP Command Line, or set the "PGP_HOME_DIR" environment variable to specify another home directory location. Some customers use a "neutral" location (e.g. C:\data\pgp) for the home directory used by all users, while others will have users point to the "home directory" of the user who initially licensed PGP Command Line. You will also need to make sure that permissions are set so that all users can read and write the preferences and keyrings.
Note that if you move the preference file and keyring files to another location, you will need to edit the preference file to change the keyring paths to the new location.