In order to use "Warning," the user has to have not been activated by ARO and either needs to be the target of an attack or within ZoC of someone who was. If a model shoots at a member of a fireteam, and they only had LoF to that one member, the rest of the fireteam will likely get no AROs - in that case, can the rest of the fireteam use Warning or did they technically Idle or something and thus can't turn around?
Fireteam member's AROs are definitely determined/triggered separately from each other, they are simply restricted to all making the same ARO declaration if they wish to retain Fireteam Integrity.
I found no hint on the Fireteam having any effect on Warning! As a result: a trooper who is in the same Fireteam as the one targeted by an attack, but out of his ZoC, cannot use Warning! Did I get that correctly?
Yep, due to the ZoC aspect. Fireteams do not alter Warning at all so it would follow the normal rules.
Interesting. Someone at a recent tournament tried to tell me I couldn't; it wasn't really crucial to the game, so I let it slide. Important to know for the future though.