How do I aummon them?
When you have the max number of minions summoned. From that point, any press of the summon button on a corps will pop a priest. It is only visable on the screen of 1 second or so as it is not a physical minion, just a heal skill in shape of a ghost.
So if you use minons, you cant have too many spells that allso consumes corpses.
(golem is not counted as a minion as it is its own seperate summon/skill)
In addition to 15% healing over 5 seconds, it also raises the damage of your minions by 30% for that time. Golems also benefit from that effect.
If you summon a priest and quickly navigate to the golem skill tooltip, you can see, how the damage numbers change and go down, when priest buff ends. Also he gets a short eerie glow, when priest is summoned.