Usually when you are in a group or there are riders behind you, you will lower your arm and point down to debris or hazards that they might not see (since they are close to your rear wheel). The lowered arm works well to warn riders behind you for a multitude of dangers ahead. You should also use the right turn and left turn hand/arm signals. Since most of the danger in group rides are the riders behind you, you need to signal them the most. Other verbal signals to riders ahead of you is telling them you are passing them, usually a "On your right!" or "On your left!" will let the riders ahead of you know where you are and where you are headed. Hope that helps.