At Delta its done by pass level and seniority. Basically we just list on the flight, then head to the airport. Being cleared from the standby list is done by pass level and seniority, with S2 being the highest-though these are limited, I believe due to my dads seniority we receive something like 10 to 20 or so each year, per person. We can fly on all Delta connection flights, and most codeshares. Some of the codeshare partners do have restrictions, such as with AirFrance where we can only sit in coach.
As long as the employee is dressed appropriately, they can be seated in first class. There are no extra charges that come with the flight card..
Delta does offer a thing called buddy passes which we can give to a friend or family member, which come with a fee- I believe 10% or so of the full fare.
They also offer benefits to domestic partners- I believe they are just like the benefits for a wife. Also with the flight card, should you be single, you can sign one person up- a friend or family member-that can use the card just like you can.
And I believe for business travel, regardless of your seniority, you will get on as long as there is space available. Though I am not sure as neither of my parents have ever had to do this, so maybe another Delta employee would know more details on business travel with the company.
There are number of other things, but I wont bore you with the details as they are getting way off what you asked.
P.S.- I may be off on what the pass level is called at Delta by some missing letter- I know it is S2 but may be missing a letter like C or something.