A Journey to Rhosgobel actually has an interesting scenario where you can add Athelas to the staging area. Is it guarded when added by Forest Grove?
No, it is not guarded when added by Forest Grove.
According to the FAQs, on guarded:
Surge, Doomed, and Guarded keywords should be resolved any time the card on which they occur is revealed from the encounter deck, including during setup.
The word revealed here is not mentioned in the actual rule book and is the cause for confusion. If we assume that Guarded is only triggered when a card is revealed then Athelas enters the staging area unguarded when added by Forest Grove.