Different airlines do things differently. With CO, the employee can designate a "travel companion" who gets their same travel privileges except at lower priority than actual employees. Boarding is determined by priority classification (employees, companions, buddy passes, etc, etc.), then seniority within each class. Companions carry the seniority of the employee. With AA, boarding is first-come, first-served. (Although priority classifications also come first there, as I recall.) CO also has discounted confirmed airfares for employees, but I've never seen one better than regularly available fares (and usually they're much worse).