Sometimes, for some reasons, especially on a PvP-realm or in a battleground, you might want to be able to be invisible.
A rogue can use Shroud of Concealment. An ability from level 76: Extends a cloak that wraps party and raid members within 20 yards in shadows, concealing them from sight for up to 15 seconds.
But first, it is from level 76, second, it lasts for 15 seconds maximum and third, it has a 5 minutes cool down.
If you want to turn invisible to surprise your enemies, there is a neat trick. You will have to use a druid who has the Glyph of the Stag (the druid’s travel from can be used as a mount by party members). This will only work for a Night Elf druid.
– Druid switches to travel form.
– Druid uses Shadowmeld.
– Party member mounts the druid.
Both will be invisible until the druid moves.