Create a blank mission with one playable WEST unit and one playable CIVILIAN player. Put an init.sqf file in the mission folder with one line of code, <player groupChat "Testing";>. Copy the mission to MPMissions folder and start a multiplayer match (I used LAN), first play as the WEST character then try again with the CIVILIAN character. Note that the same command is running on both units. If you want, you can also add <hint "Testing";> and then that should display for both.