Crowds are relatively large gatherings of people who happen to be in the same place at the same time; if they interact at all, it is only briefly.

A casual crowd

Explanation:

A casual crowd refers to a group of people who happens to gather in one place at the same time usually temporarily. This type of crowd has no similarities , bonds or long term purpose or even identity. An example is when you waiting to cross the street,a number of people will gather for a moment there and then when it time to cross you all go your separate way so it only for that brief moment that you are all there at the same time but you don't know each other usually and you will do less talking

what is the other part to the question

answer; migrate from a region;