Nothing, really. Cadet is just what it defaults to when you send the invite. In general, you should get promoted to Crewman (for random players) once you've been in the fleet a little while, participate in the projects, don't go bank-happy, etc.
For members who are also posters on TrekBBS and have long-established accounts, it should go right up to the middle, default level (officer?) after that brief "probation" window.
for the most part, we're good at sending the invites when asked, but unless you then request the promotion, no one really goes back and looks. If you got overlooked, just post in here, and I'm sure you'll get promoted.
Anything past the standard level, we don't really move around a lot. Goal is to pretty much just have the fleet admirals, and then enough Junior Adjuncts (or whatever we named them) to cover the time zones to the point where there's a chance you might find someone if you need fleet help, access to the bank goodies, etc.