Boarding order, per AS:
#1 - Pre-Boarding (special assistance, children, active duty)
#2 - First Class
#3 - Group A (Million MIlers, 75K & MVPG)
#4 - Group B (MVP, Premium Economy)
#5 - Group C (Back half of the plane)
#6 - Group D (Front half of the plane)
#7 - Group E (Saver Fares)
The groups lettering is to avoid confusion so you don't have to check multiple values (status, class, row, type, etc) printed in various areas of your ticket.
I'd say, more often than not, it's enforced, particularly in the A phase when I'm boarding I've seen people turned away. It's not difficult however to get your friend or family member to board with you if you politely state you are traveling together.
I don't get why multiple tickets on the on the same booking don't inherit the highest boarding priority in the first place.