Ok this is a short guide to flying your first multi person mount asuming you have either the Reins of the Traveler's Tundra Mammoth, or the rare VoA Mammoth drop.
Oh yes and Im asuming this only realy works for druids.
Step1: Make sure your in flightform + flying when you join the LFG system
Step2: Asuming you get into one of the dungeons you can mount up in, wait till the end and jump on your mammoth and then use the little green icon on your minimap to port out.
You should now be at your previous location still in flight form and able to fly around with your mammoth attached.
You can now give air tours of dal to the less fortunate and perhaps drop a few of them off if your feeling evil. ;D
The only downside is you can only move at 100% speed max.