Maff, Are these the only Windows 8 users in your group that have this problem? Are they showing as connected in Teamspeak?
@CBA_CO is the correct CBA mod to use.
Have your guys try to run the following in an Administrative Command Prompt:
regsvr32 "C:\Program Files (x86)\Steam\SteamApps\common\Arma 2 Operation Arrowhead\@TFAR_CO_a2\task_force_radio_pipe.dll"
If @TFAR_CO_A2 isn't in your Arma 2 OA directory, change the path between the quotes to point to where your @TFAR_CO_A2\task_force_radio_pipe.dll file is.
Just a note, they will need to do this everytime after they restart their computer. I built a script that will register the TFAR plugin and start the game after, if this works, let me know if you would like a copy.