As soon as there are enough people to kill a mark, it's not "too early" anymore. Waiting for others to share is friendly courtesy and not an obligation. I hate it too when they pull instantly, but if you can't get there in 2-3 minutes after the first call it's more or less your own fault. And this is okay, because you also have to respect people's time: Nobody wants to be sitting around a target for half an hour until everyone finally decided to show up.