Spirits Guardians...must see allies?

[Wakrob]

Spirit Guardians says you can make anyone you can see when casting immune to its effects.

So if you are in complete darkness or blind or the rogue is hiding so well you cant even see him.....does that mean you can not make your friends immune?

Or is this more a Line of Effect thing?

Wakrob

[Lunali]

You must be able to see all the creatures you wish to be unaffected at the time of casting. If someone is hidden, invisible, or not present at the time of casting, they will be affected by the spell even if you can later see them.

[NNescio]

Spirit Guardians says you can make anyone you can see when casting immune to its effects.

Yes.

So if you are in complete darkness or blind or the rogue is hiding so well you cant even see him.....does that mean you can not make your friends immune?

Exactly.

Or is this more a Line of Effect thing?

No. LoE (5e calls it "Clear Path to the Target") applies to targeting and AoEs. Spirit Guardians is a self spell, and you always have LoE to yourself (barring some odd remote-sensing/possession effects). LoE also applies to the AoE centered on you; you can't affect people behind a wall even if the wall is transparent.

In contrast, the "designate to be unaffected" part of the spell only requires that "you can see" the creature(s) "when you cast the spell". They can be behind a transparent obstacle, you can be scrying on the creature or using some remote sensing effect; it doesn't matter how you are able to see the creature, if you can see it, you can designate it to be immune, even if you don't have LoE.

If you can't see the creature though, you are unable to 'immunize' it, even if you know the creature is there. You cannot retroactively make a creature immune even if you can see it later; you can only designate creatures to be unaffected only at the moment "when you cast the spell".

[darknite]

Yep. You better see all those allies you want to exempt from the spell when you cast or they'll get chomped, too. Still one of the best spells in the game.