There is a short video on the Personal Defense Network entitled "Active Shooter Response with Family Member: Staying Behind Cover". This is intended for personal defense but is also applicable to any sudden encounter for Auxiliary volunteers, in or out of uniform. Brief discussion follows.
Scenario: In the scenario, one or more shots are fired and someone you are with goes down. As presented, your first duty is to find cover immediately. Once you have a secure position, you can then make necessary tactical decisions, such as whether you can/should engage the attacker and whether you can provide medical assistance. The important thing is that you cannot help the wounded if you yourself are a casualty. If you are working in pairs, a sudden attack occurs and someone other than your partner goes down, then you can work with your partner to establish covering fire and get to a wounded teammate or bystander if the situation calls for it, but your initial reaction which can make the difference between life and death must be drilled so that you can do the right thing instantly and instinctively.
Communication: The one thing this video does not discuss is communication. As soon as you get to a secure position and can survey the situation, you should be thinking about how you can safely and effectively call for backup. For one thing, if you are standing there with a gun after shots are fired, there becomes a likelihood that you may be mistaken for the shooter. Prompt communication can keep you from being a friendly-fire casualty when help arrives. In most places, you should use Law Mutual Aid on your radio to coordinate with law enforcement and the MTAC channel to coordinate with the larger response (e.g. medical assistance). Of course, 9-1-1 is always a safe bet as well.
What to communicate:

• Who you are (e.g. your county callsign if you have one)
• That a shooting has occured and whether the shooter is still active
• Where you are
• Emphasize that an armed friendly is on the scene and how they can identify you.
• What you need from them!

"This is Lawrence County XXX. Shots fired. Shots fired. Man down. At King's Crossing and Labrador Lane, East side of intersection. Shooter is active. Armed friendlies are present in Auxiliary uniform. Armed friendlies are present. Need immediate peace officer and medical assistance. Over."
Remember to give dispatch or a nearby unit enough time to respond before trying to repeat the message. Under stress, the wait time can seem an eternity, but keying the transmitter too soon can stomp on the transmission of someone trying to help you. Force yourself to count if you need to.
 
Drill: This basic exercise is one you drill in your dry fire practice (with or without a partner). Practice falling into your stance, making a proper presentation from holster while moving to cover, and surveying the situation from your chosen cover. Learning to move your feet while looking and thinking is critical.
Fall Back On Training: In any emergency situation, the first step in response is always size-up: What is the situation? How bad is it? Is the danger over? Followed by: What is the appropriate response? What does my training say to do? It does not really matter whether it is a shooting, a traffic accident, or a stampede. If you cannot keep yourself out of danger, you cannot perform the size-up, so immediate personal safety is step zero and must become a trained and practiced response. Falling back on trained response controls the stress which may cause you to freeze up and become a casualty yourself.