Humans are social creatures. We have a propensity to seek out others and engage in interactions of all sorts.
This is the underlying instinct that drives people to look for like-minded people to "group up" with.
There's a problem with this instinct, however. If followed without caution, it allows for easy predation. This can lead to underhanded people taking advantage of unsuspecting victims.
Before you begin to engage with someone on any level that could pose a direct threat to your safety in the long run, you must really get to know the person. That's why we cherish conversations during social interactions so much.
You need to feel people out, not just first impressions or idle chit chat, but with long term relationships where you get to know the person's background and understand their plans for the future.
Only then should you open yourself up to sharing things such as your stockpiles of food, or your plans for surviving a harsh winter, or your opinions on a pending economic collapse, or your security protocols and defense strategies.
Only when you can truly trust a person should you even consider "grouping" with them to form a prepper community.
Without this relationship, you're risking a lot. You could have all of your preps stolen out from under you. You could entice unwanted visitors to show up and expect to be taken care of. You risk one person telling another person about your plans and preps, and having zero control over how that 3rd party deals with that information.
At the very least, you could end up with a "group" of people who don't really know or trust each other, and that can cause its own issues with internal power struggles or rumors, which can dash any benefits such a group may provide.
The topic of forming a "group" is not something I personally think is a good idea to advertise.
Instead, you need to start your own organic group. This would consist of close friends, family, and neighbors you've had long relationships with who have a shared interest in maintaining security in your local area.
Strength in numbers is always good, but you must be able to trust your numbers, or you'll only have the illusion of strength.