In a 5vs5 is not that easy because 5 snipes in a row can kill most dg from the start.
How do you counter it?
Even if they are not in voice they can do it just fine, one manage the target and write down "rook" or "erebus" in chat and everyone just snipe = kill.
Well, first off if this comes up EVERYONE should be getting 800 hp favor item and 400 hp item, along with healing potions. Ideally you will be playing a custom game and can play DGs who are very strong against this tactic, including QoT, Sedna, Oak and Rook. At this point there are no instant kills, and shouldn't be for the entire game. Continually upgrading HP above others will make sure this happens. Generally one person will be targetted as the "weak link" and will be focused upon, or players will call out low hp DGs. Either way, it's gonna be rough times for some people, and the Regulus' are probably going to have more kills. That's fine, because you have all their flags and are simply trying to limit their early game kills. Remember, they don't have mana to continually snipe anyway (unless they just sit at base, which is a whole new level of noobness), so your losses aren't going to be devestating.
Since they are going snipe it's unlikely they are going mines, upgrade citadel to cats, use much higher pushability and win.
But you are right, I have only done this in a 4v4, not a 5v5. The 4v4 was a regulus rapefest, but they were not perfectly coodinated.