I'm a bit late to the party but I will respond in two categories, My experience of being inside the vehicle and rolling and Responding to a roll over.
Being inside,
I have been flopped on the side many times but only had a few hard flops/rollovers that were big. I only have one flop under my belt that I knew was going to happen and nothing I did would change it. In that case I stopped, shutoff the buggy, talked with my passenger and then fell over. It was slow, very easy flop. Recovery wasn't bad and having a chance to "discuss" what was happening was nice but a rarity. Every other time I've rolled has been something that I drove until the absolute last second that I could. There is no preparing and thus your self preservation instincts kick in and those are somewhat uncontrollable but also manageable.
As either a driver or passenger: keep your hands and feet inside, try not to "catch" yourself by putting your hand up or out. My wife has a damaged pinky nail from putting her hand on the roof during a 1.75Xs tumble down a steep obstacle after reaching up and getting it caught between a cage bar and the roof skin. Stay calm, I rolled over easy with my 7 year old daughter in the buggy this fall, she really wanted to freak out but the fact that I treated it as just another fun part of the day kept the crying and freak out sort of at bay. When the vehicle stops moving get it shut off, in park, and verify any occupants are OK. If possible call out that everyone is OK in the vehicle, and as was pointed out earlier in the case of Stephen rolling in the hot tub any issues you know of. Keep calm, make sure the scene is safe, unbuckle the down hill passengers first, then uphill second. Be prepared to hit your head if upside down, get out of the vehicle and administer any first aid as needed, it takes some time for nerves and adrenaline to allow someone to know they are hurt. In the case above it was at least a minute or more before my wives finger began to even bleed and even longer before she even felt it. Don't be surprised if minutes later someone goes into shock etc.
Being Outside,
As an observer pay attention to where you are, where others are and what your escape routes are. Look at the lady that was ran over at EJS on Golden Spike as an example of freak things happen and vehicles can travel distance fast. If your not going to pay close attention and just want to chill and hang out get farther away so that the lack of paying attention is safer. That said anyone with basic first responder training will recognize the following;
Try to watch closely to see if anyone sticks a hand or leg out or comes loose from the seat belts, Once all motion is done verify scene safety, don't enter an unsafe scene, try to establish voice contact with occupants, Provided the scene is safe approach the vehicle. At this point things become very dynamic, is there an issue that needs to be dealt with besides basic recovery, injury, fuel leak, battery issue, vehicle stability issue etc. These need to be assessed and dealt with, again secure the scene as a priority. DO YOU NEED TO CALL FOR HELP? Establish this early if there is an injury, it takes time for first responders to arrive. As pointed out above about the fuel leak, Stephen had it called out and was able to get a rag over the leak pretty quick, We called for fire extinguishers and for everyone not needed to stay back. Scene safety!!! Once vehicle/scene issues are addressed and occupants are either safe or being treated away from the vehicle it's time to focus on recovery. Again recovery can be a pretty dynamic situation, but paying attention to winching directions, attachment points to the vehicle and how those are going to move the vehicle, do you need two winches for safety etc. One in charge person as pointed out above, make sure the vehicle is in park/secure and when it does set down on it's wheels it can't keep going.
Cliff notes: Stay calm, try to stay inside the vehicle, secure the vehicle, administer first aid as needed, call for help if needed, stay calm, get vehicle upright.
I'm sure there's holes all over in this but having been through a some amount of first aid training in the last few years as well as years of doing dumb things and rolling vehicles over I feel somewhat uniquely qualified at an amateur level to give advice.