Just like the Dupe glitch, you need a friend to become invincible too...

You need to first create a full set of Diamond Armor and then place that in your inventory.

Then place a Dispender and put the armor in the Dispenser.

Your friend then need to destroy the Dispenser.

After that, grab your Diamond Armor back and put it in your inventory. Then equip it.

Finally, your friend needs to hit you. The armor will disappear, but you will be invincible!