This just shows how anything can happen to anyone. When meeting up with strangers for the purpose of transacting a purchase or sale of a firearm, people need to understand that they are taking a risk. I myself have done it several times without a second thought, but I always prepare for the worse, as best I can. I would not meet with 2 people ever. If they showed up like that I would drive away and call or email the guy later.
There are only a limited amount of things one can do, like meet in a gun store or other such place, but sometimes we are all in a hurry and do stupid things. Try to get as much information about the person before hand as you can, and call them on their number as well as email them on their account, it's far from foolproof, but at least you have a reference point. You can also do a backwards trace of the phone # online to see who it belongs to, as well as their email and associated facebook and social media accounts.
I don't think that bringing people along is that big a deterrent, most robbers would be happy with more targets to rob.
Otherwise keep it public gun show, gun store, range, etc.