Seems to be just single player.
Invisibility is mostly PvP oriented in a way, but it works against monsters unless you bump into them. Now here's the problem: if you're wearing armor, mobs can see you from farther away, for each piece of armor you put on. (no armor you have to bump them, 2 pieces is about 6 blocks, and 4 pieces is more than 10 blocks away).
Greg does something with his cloaking device (chest armor) that makes all other pieces of armor invisible as well, but doesn't reduce the sight radius of monsters, so it's almost purely PvP. holy derp transparency module batman