Note that this will not spawn additional Lunatics if you already have a Nagelring equipped by either you or your hired follower (more than 4). So if you have 4 Spawned Lunatics, more will not spawn from this bug.
So you can only do this in town at least from what I see.
1) You have to make sure a Nagelring is equipped with another follower (not the one you have hired).
2) In town go the follower that has the Nagelring equipped.
3) Choose to "View Inventory" of that Follower.
4) "Mouse-Over" the Nagelring to show the item details popup.
5) Just wait and the Lunatics will Spawn!
Once you have the 4 Max Lunatics you can close the follower window.
These will follow you as if you spawned them with an equipped ring and will explode the same. No more will magically spawn as you don't have the ring on.
Once again, this is a Bug. Obviously if you want Lunatics to spawn all of the time, just equip the ring yourself or have your hired follower equip it.
If you go into a Trial Rift after spawning the free Lunatics, they will explode and vanish when you enter the portal.
Enjoy!