In order to implement something like this in BigFix, you first have to determine how you would do it manually/normally, such as from a command line. This is what Jason was asking above. Once you have that script or set of commands, then you can proceed to implement it in the Action tab of a Task. There are many examples of executing a Linux script on this board.
You will probably have to stash the results of the command execution in a file, which will be read by an Analysis, so you can get that list of users back to the BigFix Console.
If you post the script here, others on the board can be of more assistance to you.