To add to this, in England we would pretty much never say 'hi all', that would seem quite unnatural. I concur thought that Hello everyone would be used in a presentation environment, where you don't know all the individuals. Again though in England we might use hi everyone amongst friends/acquaintances to greet the group but it's not a sufficient enough of a greeting so we would then go on to greet people we have a closer relationship with using phrases like "how are you?", "how's things?", "what's happening?" "Are you ok?" etc. confusing I know, but in an inforal setting saying hi everyone is good as rule and then further greet people you have a closer relationship with.