During MP games, it's often handing to place markers on maps for teammates to read and make note of. BIS' Suma reminds us all that markers placed on the map during MP games are visible to those able to listen on the radio channel currently selected for transmission by the player placing the marker. If you want to place a marker that only your teammates will see, select the SIDE channel. If you want to place a marker for your group only, select the GROUP channel.
BIS forum moderator and postcount dropper Ralph Wiggum reminded me that, in multiplayer games, you can select from a variety of markers available by pressing the SHIFT and UP or DOWN arrow keys simultaneously.
To remove a map marker, just place your cursor over it and press your keyboard's DELETE button.