X minutes before a scheduled event starts a reminder popup should be displayed for all members who chose so (through an option).

The popup might come from Steam (ArmA Units - Steam interface?) or might come from ArmA game itself (ArmA Units - ArmA game interface?) or might come from the ArmA 3 Launcher (that should be setup as a startup program running silently in the background and displaying an overlayed popup just like Steam).