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?