They create a single-file queue, with what is supposed to be a strict linear sorting. Instead of a group, you get a boarding number from A1 through C60. There's some flexibility in this, generally most people don't mind #44 going before #40, but you won't be able to line up or board after person #11 if your boarding card says #56. The gate agent will make you go to your correct place in line. Since it's a single-aisle plane, they run everyone through a single ticket reader, and thus have a point of enforcement.
Yes, and they also have these pylons next to the gate, with numbers on them that are for forming the line. During boarding, everyone stands next to the pylon with their number on it and thus the line is formed. Southwest gates are unusual in this way. No other airline has these pylons at their gates.
I remember one gate agent who was unusually vocal about everyone obeying the rules. It was pretty funny actually.
