In zombie apocalypse stories, people are generally more likely to survive if they seek power in numbers. It’s easier to fight off a herd of the walking dead if you have a dozen people by your side. That means you can’t disregard other people and face-to-face communication is a necessity — a stark contrast from today’s world, in which cellphones and computers allow people to communicate without ever meeting in person.
Mass hysteria has a way of showing its face whenever tragedy strikes. It has been said that a person is smart, but people are idiots. This is a sad reality when it comes to catastrophic events.
The major advantage of having a large group is manpower, making the group more capable of fending off large swarms of zombies. In this way large groups can be considered safer, though, as mentioned above, there are drawbacks.
Bigger groups will eat up food supplies much faster, making large groups less effective in a siege situation. Secondly, larger groups will attract more zombies. Again, this will make siege situations much more difficult.
Once the word is out and zombies begin seriously popping up, small groups will begin to form. It’s a sort of ‘we’re stronger in numbers’ mentality. This is very true, but it is important to survey a group before joining. You may be better off on your own than in some groups.
On the other hand, you want to make sure that your group is up to par with what is required for survival. If you have a bunch of uneducated, non-confrontational, zero survival mentality individuals in one group, that group’s chances of survival are significantly reduced.