RaidBuddy

This addon will let you set members of your party/raid to various marks.  Then with a single click it will notify your group who is responsible for which marks.

Open with the floating movingable icon or you can hotkey it (using WoW's Options > Keybindings > RaidBuddy)

With the window open, specify each character responsible for which mark.  This will persist through open/close however it will not persist through a /reload or restarting of the game client.

Once your marks are set with character names, push the relevant communication button - party, raid, or raidwarning.  That's it!