FHQ MapDraw is a tiny addon that hooks into the drawing code of the map (briefing or in-game) and allows players to draw lines on the map that are shared among all players. It is useful for Co-Op games for laying out attack routes, marking things, etc.
It works both on the briefing screen (see also the bugs section of the manual) as well as on the normal map. Holding the Control key will start drawing a line, successive clicks will extend that line until Control is released. The README lists more keys that can be used to modify the lines being drawn.
This addon is mission-independent, it works in every mission and doesn’t need any modification.
The Community Base Addon (CBA) is required for this to work.