I found this by accident before "a griffon tamer" patch and it still works. Hail the griffon tamer but don't choose a destination yet. Then, go into stealth mode/invis mode. Then select the destination. When you mount the griffon, it will turn invis with you!
Also for you scouts with the Smuggle skill, if your timing is right, you and a group member can click simultaneously and both have Stealth Griffons (be warned though, Stealth Griffons get real naughty when they know no one can see them)!
The griffon also shares the same effects of some spells like faerie fire and that purply damage shield. It's great fun to watch!
Arre
