The description of the spirit guardians spell says, in part (emphasis mine):
When you cast this spell, you can designate any number of creatures you can see to be unaffected by it. An affected creature’s speed is
halved in the area, and when the creature enters the area for the
first time on a turn or starts its turn there, it must make a Wisdom
saving throw. On a failed save, the creature takes 3d8 radiant damage
(if you are good or neutral) or 3d8 necrotic damage (if you are evil).
On a successful save, the creature takes half as much damage.
Let's say the caster is on the second floor and he casts spirit guardians. He designates his ally next to him to be unaffected by it. Now, directly below him, two other allied PCs are waiting; however, since he can't see them, he can't designate them to be unaffected by the spell.
Will the players on the bottom floor be affected by the spirit guardians spell once they go up to the 2nd floor?
Can the caster decide at some point after casting the spell that the spell won't hurt them? When does the caster need to decide who is and isn't affected?
Best Answer
RAW, to be unaffected a creature must be seen when the spell is cast.
The RAW ruling here is pretty clear. The relevant portion of spirit guardians' spell description says (PHB, pg. 278):
You must be able to see the creature when you cast the spell to be able to designate it as an unaffected creature.
In your specific situation, the characters that were one floor below would be affected by spirit guardians upon entering its area of effect, as the caster could not see them when they cast it to be able to designate them unaffected.
Rules-as-fun may call for a different ruling - discuss it with your table.
I've played in games that ruled this both ways. At one table, we were all very committed to rules-as-written, and were aware of this feature of the spell. We had no problems with ruling the RAW way here, as we were all aware of its tactical implications before it was ever used. It made for an epic scene when our raging barbarian ran head first into the spirit guardians to save the cleric who was engaged with a manticore that had surprised us, tanking the damage to save her friend.
At a different table, most of the players were less experienced, and our cleric was rather disappointed when the DM suggested the RAW ruling. We took a minute to talk about it, and decided that we go with "no friendly fire" for spirit guardians, and that worked just fine for that table.
If you're a DM and one of your players has spirit guardians, talk it through with them before you have to rule on it in play - it can be an unpleasant surprise if you haven't thought through the details of the description and how it plays out tactically.