Once the Mac is encrypted, you can add / delete a user using the following fdesetup commands. You will not be able to add groups.
In all cases where you're adding a user with fdesetup, you will need to have the username and password of both the account you want to add as well as the username and password of an FileVault 2-enabled account already on the Mac in question.
To add: sudo fdesetup add -usertoadd username_here
This is an interactive process with prompts. You would need to provide both the username and password of a previously enabled account as well as the password of the account you want to add.
If you want to use a non-interactive script, you can set up a plist file with the information you want to use and have fdesetup import the plist.
sudo fdesetup add -inputplist < /path/to/filename.plist
The plist would need to follow the format available at the following link: http://pastie.org/4673195
When adding additional users using a plist file, the top level Username key is ignored, and the Password key value should either be an existing FileVault user’s password or the recovery key. Additional users can be added as needed by adding additional user information under the AdditionalUsers plist key.
Note: All account passwords need to be supplied in cleartext.
You can remove users from the list of FileVault enabled accounts by using either their username or the account’s UUID. When removing accounts, you do not need to provide account passwords.
To remove the account using the username, run the following command:
sudo fdesetup remove -user username_here
To remove the account using the account’s UUID, run the following command:
sudo fdesetup remove -uuid UUID_here
You can get both the usernames and UUID by running the following command:
sudo fdesetup list
The User Accounts listing in the machine's inventory on the JSS should also show which user accounts are enabled. If enabled, the FileVault Enabled line should show as true.