Wholly owned express pilots are A3 and other express carriers are all lumped into A4. But like the previous post said, if there's seats in the cabin you all get on.
Between two wholly owned guys the pilot who is more senior gets on, not sure about between two air whisky guys.
Just to clarify, you were referring to mainline flights? If so, the previous posters are correct. If you are jumping on an express flight, it becomes more complicated, as it depends on who is doing the flight, who the ACM works for, etc.
As a WO commuter who has bumped contract pilots of the jump seat at mainline, I can tell you WO pilots have priority over contract pilots on mainline flights, even J4J guys. Mainline pilots are the only ones that have priority over WO pilots. If there are no mainline pilots (mid-atlantic pilots are considered mainline), then it is first come first serve among WO pilots. Sorry, but after being shaat on for so many years, it is one of the few perks we have.
On a related note, a PSA pilot has priority over a mainline or contract pilot on a PSA flight. I ran into a little problem in CLT the other day... I listed as a regular non-rev for my commute to TYS on a PSA flight. I was told by the agent that the flight was full and that I should fill out a jumpseat form just in case. No problem. But I guess when she took my jumpseat form, I was deleted from the non-rev list. Therefore, when boarding began and non-revs were called up in priority order, she called up a CHQ pilot. Meanwhile I was standing there wondering why she hadn't called me yet. I was a little nervous because I knew there was a weight issue and that the jumpseat might not be available. The problem resolved itself when there ended up being a seat open for me. But there could have been a real problem if there were no seats left and the jumpseat was unsuable due to weight. I was not looking forward to having to argue with the gate agent and possible bumping the CHQ pilot who had already boarded. Keep an eye on those agents and politely remind them when they are not following the correct priority procedures.
...and a CHQ pilot has priority over a mainline or WO pilot on a CHQ flight, a Mesa pilot has the same on a Mesa flight, a TSA pilot the same on a TSA flight, let's see... who am I forgetting. Well, we all get the picture. Thankfully, ,the vast majority of us will do everything we can to get anyone and everyone aboard.