I don't think there will be large groups/militias out raiding preppers..... What does it take to have a group like that? For starters, a lack of ethics. When you group together people of questionable character, it is just a matter of time before they turn on each other.
But, lets say such groups did organize... They would have to be continuously on the move as they deplete the resources immediately available. These people are not producers, they are consumers. When there is nothing to consume, they will move on, and when they do move on and encounter ever decreasing resources, they will still turn on each other because that is their personality, that is what brought them together in the first place. I would be terrified to be a part of such a group.
It is far more likely that people will group together to produce what they need. This too has it's problems, and most of them are rooted in socialism. Your group has a doctor, a gardener, a defense contractor, a few hunter/trappers, a mechanic, and if you are lucky, a good scientist. Problem is, the doctor will not always be needed, a garden might be under snow, and there will always be people retaining the notion that their responsibility is well defined and that their presence as a part time consumer is invariably justified. I would like to believe that these uniquely skilled people will take on other tasks when their most notable skillset is not being utilized, but when you are the only one that knows how to remove a bullet - but there are presently no bullets to remove, it is easy to feel valuable enough to sit on your ass. Any group situation like this will default to some level of socialism. I would prefer to keep my location limited to the family and depend on hard work and bartering with locals doing the same. Being a witness to the shortcomings of the human psyche, I have completely abandoned the idea of active group prepping. Marauders thinking they will succeed long term will eventually grow hungry, and hungry people are physically weak, slow, and emotionally compromised.