It was all three GD members plus 2 other guys. They tried to hide their identity by putting ski masks on (even during interviews). The bad thing is Billie Joe tried to even hide his voice by singing like a robot about robots. Plus the music wasn't even close to green day. Go to their website, they have some samples I think or if not try amazon for samples or something. I'm still angry about it.
They were under Green Day's record label, they are the same physical size and there is close up pictures of them (with the masks) where you can see 2 inch diameter eye holes and a mouth hole which is enough to see that its them (Billie Joe has very destinct eyes). I bet it was just them having fun, but still, it was aweful.