It would be very useful addon that would display a message from the game in to Windows that your group are ready. Coz search for a group for DD takes a huge amount of time, and many, I'm sure, fold the game while searching group for a dungeon. Such an add-on will not allow you to miss the time of the group gathering in Alt+tab. Some sound will be enought.
I think the sound could be maybe possible but writing something to the windows screen, or changing autoamtically back to the game, wouldn't be possible by the help of addons!
-> Not sure if there is an event fired if the group is ready
The sound will be only heard if you enable the ingame sounds to be still active if you switch out of the game though.