Comment by whorleater

At least for the major US airlines (Delta, United, AA):

- Group 0/pre-boarding is active military, disabled, and infants

- Group 1 is first class & mileage club people

- Group 2 is business class

- Group 3/4 is usually economy

- Group 5 is usually the cheapest basic economy people

You can usually get bumped up to Group 2 by getting the airline's credit card or having a bunch of miles