Yeah, it's hard on your battery. There's a battery saver mode that helps a bit. When the phone is upside down (in a pocket or held normally in hand while walking), it powers down the display. But it's still hard on the battery because it's running a lot of processes to track location, ping servers, display graphics, etc. You don't really fight your friends in this game, as it's better for your friends to play cooperatively. For example, you and your friends are Blue team. You would go out and pursue gyms held by Red or Yellow teams and defeat them together. If you're all different teams, it doesn't encourage playing together, as you'd often be directly competing for the same stuff (and this game, like Ingress, is all about alliances... 1-2 people often can't take a gym... But 3-4 can).