I was curious for anyone wanting to help others who would and should have to help out what security measures do you take to make sure that they can't make off with your food and other stores in the middle of the night or worse yet bring in others with the same goal?
Certainly a valid question. I'm all for helping others, as I think that in such a world, the extra personnel are simply added defense...but as they are also an added burden, they'll need to pull their weight to make them an asset vs. a liability.
Security wise, you limit the number of newbies to only that which you can feasibly provide for immediately, and to those you can guard at night. We'd all be sleeping in shifts anyhow, but newbies would have to be supervised at all times (even when sleeping). There would be an evaluation period, to ensure they liked it here, we liked them, etc.
As for them bringing in others, newbies would be supervised, so wouldn't have the chance to signal, communicate, etc. Also, you'd want scouts and other ways to know what's going on around you, to at least detect some large incoming group, prior to them really finding you.
This is really going to be the major SHTF challenge, trusting other humans, and balancing that trust with security measures.