One frequent safety complaint I receive is from riders being passed dangerously. Please avoid passing on a rider’s right side whenever possible! Passing on the right side often startles a rider and most riders’ first reaction when startled is to move right. Communication is the key. Let the person in front of you know when you are passing, but especially in the rare instance when you must pass on their right. 

When riding with a group, position your bike directly beside or behind your companions. Do not partially “overlap” your wheels because a sudden change of course would cause the trailing bike (and probably others) to crash. For a vivid example of what can happen, see this short YouTube video The Danger of Overlapping Wheels.