These smells are typically caused by sulfur bacteria or sulfate-reducing bacteria. While not harmful, this can be a very unpleasant condition. We recommend to first disinfect the well (PDF) and the water system thoroughly.
Be careful to use a strong enough solution and to make sure the chlorine water gets into every portion of the water pipe system. After flushing out the chlorine, if the smell returns, you may want to apply to our office for a sulfur bacteria sample.